realtek: remove redundant is_lagmember checks

realtek: remove redundant is_lagmember checks

All callers of the rtl83xx_mc_group_* functions already do the same
check, so these aren't needed.

For rtl83xx_mc_group_alloc, this branch also incorrectly returned 0
instead of a negative value. If the branch wasn't effectively dead code
anyway, this could potentially have caused bugs, as 0 is a valid
multicast group entry index.

Fixes: cde31976e375 ("realtek: Add support for Layer 2 Multicast")
Signed-off-by: Jan Hoffmann <[email protected]>