if_vlan: fix kdoc warnings

While merging net to net-next I noticed that the kdoc above
__vlan_get_protocol_offset() has the wrong function name.
Fix that and all the other kdoc warnings in this file.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250106174620.1855269-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>