cdc-ether: clean packet filter upon probe

There are devices that don't do reset all the way. So the packet filter should
be set to a sane initial value. Failure to do so leads to intermittent failures
of DHCP on some systems under some conditions.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>