Add comments about difference between Airoha AN8855 and Mediatek tag
bitmap.

Airoha AN88555 doesn't support controlling SA learning and Leaky VLAN
from tag. Although these bits are not used (and even not defined for
Leaky VLAN), it's worth to add comments for these difference to prevent
any kind of regression in the future if ever these bits will be used.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
---
 net/dsa/tag_mtk.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/dsa/tag_mtk.c b/net/dsa/tag_mtk.c
index b670e3c53e91..ac3f956abe39 100644
--- a/net/dsa/tag_mtk.c
+++ b/net/dsa/tag_mtk.c
@@ -18,6 +18,9 @@
 #define MTK_HDR_XMIT_TAGGED_TPID_88A8	2
 #define MTK_HDR_RECV_SOURCE_PORT_MASK	GENMASK(2, 0)
 #define MTK_HDR_XMIT_DP_BIT_MASK	GENMASK(5, 0)
+/* AN8855 doesn't support SA_DIS and Leaky VLAN
+ * control in tag as these bits doesn't exist.
+ */
 #define MTK_HDR_XMIT_SA_DIS		BIT(6)
 
 static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
-- 
2.48.1