mlxsw: spectrum_fid: Remove flood_index() from FID operation structure

The flood_index() function is not needed anymore, as in the unified
bridge model the flood index is calculated using 'mid_base' and
'fid_offset'.

Remove this function.

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>