mlxsw: Remove lag_vid_valid indication

Currently 'struct mlxsw_sp_fid_family' has a field which indicates if
'lag_vid' is valid for use in SFD register.

This is a leftover from using .1Q FIDs instead of emulating them using
.1D FIDs.

Currently when .1Q FIDs are emulated using .1D FIDs, this field is true
for both families, so there is no reason to maintain it.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>