USB: cdc_ether: remove unneeded check

We already verified that "dev->udev->actconfig->extralen" was non-zero
so "len" is non-zero here as well.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>