Currently, this API is only guarded by CONFIG_OF. But the function
definition is guarded by CONFIG_SERIAL_DEV_BUS symbol in the .c file. This
causes below build error if CONFIG_SERIAL_DEV_BUS is not selected but only
CONFIG_OF:

pwrseq-pcie-m2.c:(.text+0x924): undefined reference to `of_find_serdev_controller_by_node'

Fix this issue by adding the CONFIG_SERIAL_DEV_BUS guard to the function
prototype.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604011226.KGNn5974-lkp@intel.com/
Fixes: a2b4814190af ("serdev: Add an API to find the serdev controller associated with the devicetree node")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
---
 include/linux/serdev.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/serdev.h b/include/linux/serdev.h
index 0de261a26284..58f000534bdb 100644
--- a/include/linux/serdev.h
+++ b/include/linux/serdev.h
@@ -356,13 +356,13 @@ static inline bool serdev_acpi_get_uart_resource(struct acpi_resource *ares,
 }
 #endif /* CONFIG_ACPI */
 
-#ifdef CONFIG_OF
+#if IS_ENABLED(CONFIG_OF) && IS_ENABLED(CONFIG_SERIAL_DEV_BUS)
 struct serdev_controller *of_find_serdev_controller_by_node(struct device_node *node);
 #else
 static inline struct serdev_controller *of_find_serdev_controller_by_node(struct device_node *node)
 {
 	return NULL;
 }
-#endif /* CONFIG_OF */
+#endif /* CONFIG_OF && CONFIG_SERIAL_DEV_BUS */
 
 #endif /*_LINUX_SERDEV_H */
-- 
2.51.0