mlxsw: spectrum_router: Add rif helper functions

Add rif helper function to access the rif index and rif devices ifindex.
This functions will be used by dpipe in order to dump the rif table.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>