mlxsw: spectrum: Add set_rx_mode ndo stub

Add just a stub for now. This allows to pass check in dev_ifsioc,
SIOCADDMULTI and SIOCDELMULTI cases. Teamd is using these to add LACP
slow MAC.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>