mlxsw: spectrum_router: Place RIF related code with router code

The inetaddr notification block is currently implemented in the main
driver file, but this isn't really appropriate, as it mainly creates and
destroys router interfaces (RIFs) which belong with the rest of the
router code.

This will become even more apparent later on when we'll need to bind
these RIFs to virtual routers according to the VRF's table.

Structure the driver better and prevent unnecessary function exports by
moving the RIF related code with the rest of the router code.

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>