cdc-ether: divorce initialisation with a filter reset and a generic method

Some devices need their multicast filter reset but others are crashed by that.
So the methods need to be separated.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by: "Ridgway, Keith" <kridgway@harris.com>
Signed-off-by: David S. Miller <davem@davemloft.net>