mlxsw: spectrum_fid: Use 'fid->fid_offset' when setting VNI

The previous patch added 'fid_offset' field to FID structure. Now, this
field can be used when VNI is set using SFMR register. Currently
'fid_offset' is set to zero, instead, use the new field which is now set
to zero and in the future will be changed.

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>