mlxsw: spectrum_router: Allocate RIF prior to its configuration

In the following patches the RIF's configuration function is going to
expect a RIF struct with all the necessary information.

Therefore, allocate the RIF just before it's configured to the device.

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