mlxsw: spectrum_router: Ignore address families other than IPv4

We're about to add IPv6 notifications in the FIB notification chain, but
the driver currently doesn't support these, so ignore them.

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>