mlxsw: spectrum_router: Propagate extack to .fid_get()

In the follow-up patch, mlxsw_sp_rif_vlan_fid_get() will be changed in a
way that could fail. Give that function a possibility to explain the
failure through extack.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>