mlxsw: spectrum_router: Initialize RIFs in a separate function

The router interfaces (RIFs) array is currently initialized together
with the general router configuration. However, in a follow-up patchset
we're going to introduce a common RIF core that will require us to
initialize more RIF constructs, so move the RIF initialization to its
own function.

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>