usbnet: allow mini-drivers to consume L2 headers

Assume the minidriver has taken care of all L2 header parsing
if it sets skb->protocol.  This allows the minidriver to
support non-ethernet L2 headers, and even operate without
any L2 header at all.

Change-Id: Ied7fc3752673a68e5bb22a623cc3fc59c1f1e0ff
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 81e0ce79f2919dbd5f025894d29aa806af8695c7
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[gustavos@codeaurora.org: fix checkpatch warning]
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>