mlxsw: spectrum_router: Publish mlxsw_sp_rif_find_by_dev()

In order to guard against removal of a PVID for which a FID was
allocated, spectrum_switchdev needs to first determine whether there is
a RIF associated with a given bridge. To that end, publish a preexisting
function mlxsw_sp_rif_find_by_dev().

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>