mlxsw: spectrum_fid: Save 'fid_offset' as part of FID structure

SFMR register contains a 'fid_offset' field which is used when flooding
tables of type FID offset are used.

Currently, the driver sets this field to zero, as flooding tables of type
FID are used.

Using unified bridge model, the driver will use FID offset flooding
tables. As preparation, add 'fid_offset' to 'struct mlxsw_sp_fid'. Then,
use this field instead of passing zero to the function that configures
SFMR.

Set the new field as part of 'ops->setup()', for that, implement this
function for dummy FID and rFID.

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