mlxsw: spectrum_router: Allow IPv6 routes to be programmed

Take advantage of previous patch and allow the RALUE register to be
called with IPv6 routes.

In order to re-use as much code as possible between IPv4 and IPv6, only
the lowest-level function that actually does the register packing is
demuxed based on the passed protocol.

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>