Some of the serdev APIs are not guarded by CONFIG_SERIAL_DEV_BUS and also
missing the stubs when the symbol is not selected. This leads to the below
build errors:

   drivers/power/sequencing/pwrseq-pcie-m2.o: in function `pwrseq_pcie_m2_remove_serdev':
>> pwrseq-pcie-m2.c:(.text+0x260): undefined reference to `serdev_device_remove'
   powerpc64-linux-ld: drivers/power/sequencing/pwrseq-pcie-m2.o: in function `pwrseq_m2_pcie_notify':
>> powerpc64-linux-ld: pwrseq-pcie-m2.c:(.text+0x9c8): undefined reference to `serdev_device_alloc'
>> powerpc64-linux-ld: pwrseq-pcie-m2.c:(.text+0xc00): undefined reference to `serdev_device_add'

Fix these issues by adding the CONFIG_SERIAL_DEV_BUS guard to function
prototypes and stubs when the symbol is not selected.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604011226.KGNn5974-lkp@intel.com/
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
---
 include/linux/serdev.h | 42 ++++++++++++++++++++++++++++++++----------
 1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/include/linux/serdev.h b/include/linux/serdev.h
index 188c0ba62d50..0de261a26284 100644
--- a/include/linux/serdev.h
+++ b/include/linux/serdev.h
@@ -156,16 +156,6 @@ static inline void serdev_controller_put(struct serdev_controller *ctrl)
 		put_device(&ctrl->dev);
 }
 
-struct serdev_device *serdev_device_alloc(struct serdev_controller *);
-int serdev_device_add(struct serdev_device *);
-void serdev_device_remove(struct serdev_device *);
-
-struct serdev_controller *serdev_controller_alloc(struct device *host,
-						  struct device *parent,
-						  size_t size);
-int serdev_controller_add(struct serdev_controller *);
-void serdev_controller_remove(struct serdev_controller *);
-
 static inline void serdev_controller_write_wakeup(struct serdev_controller *ctrl)
 {
 	struct serdev_device *serdev = ctrl->serdev;
@@ -204,6 +194,16 @@ void serdev_device_write_wakeup(struct serdev_device *);
 ssize_t serdev_device_write(struct serdev_device *, const u8 *, size_t, long);
 void serdev_device_write_flush(struct serdev_device *);
 
+struct serdev_device *serdev_device_alloc(struct serdev_controller *);
+int serdev_device_add(struct serdev_device *);
+void serdev_device_remove(struct serdev_device *);
+
+struct serdev_controller *serdev_controller_alloc(struct device *host,
+						  struct device *parent,
+						  size_t size);
+int serdev_controller_add(struct serdev_controller *);
+void serdev_controller_remove(struct serdev_controller *);
+
 /*
  * serdev device driver functions
  */
@@ -264,6 +264,28 @@ static inline ssize_t serdev_device_write(struct serdev_device *sdev,
 }
 static inline void serdev_device_write_flush(struct serdev_device *sdev) {}
 
+static inline struct serdev_device *serdev_device_alloc(struct serdev_controller *)
+{
+	return NULL;
+}
+static inline int serdev_device_add(struct serdev_device *)
+{
+	return -ENODEV;
+}
+static inline void serdev_device_remove(struct serdev_device *) {}
+
+static inline struct serdev_controller *serdev_controller_alloc(struct device *host,
+						  struct device *parent,
+						  size_t size)
+{
+	return NULL;
+}
+static inline int serdev_controller_add(struct serdev_controller *)
+{
+	return -ENODEV;
+}
+static inline void serdev_controller_remove(struct serdev_controller *) {}
+
 #define serdev_device_driver_register(x)
 #define serdev_device_driver_unregister(x)
 
-- 
2.51.0