cdc-ether: handle promiscuous mode with a set_rx_mode callback

Promiscuous mode was not supported anymore with my Lenovo adapters
(RTL8153) since commit c472ab68


(cdc-ether: clean packet filter upon probe).

It was not possible to use them in a bridge anymore.

Signed-off-by: Olivier Blin <olivier.blin@softathome.com>
Also-analyzed-by: Loïc Yhuel <loic.yhuel@softathome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>