mlxsw: spectrum_router: Export RIF dev access function

The mlxsw_sp_rif struct, defined as private struct in spectrum_router.c
will be used in the multicast router source file. Due to the fact that the
dev field will be needed by the multicast router logic, add an access
function to it.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>