mlxsw: reg: Extract mlxsw_reg_ritr_mac_pack()

Unlike other interface types, loopback RIFs do not have MAC address. So
drop the corresponding argument from mlxsw_reg_ritr_pack() and move it
to a new function. Call that from callers of mlxsw_reg_ritr_pack.

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