usbnet: Zero padding byte if there is tail room in skb

Usbnet adds a padding byte if a 0 byte USB packet would be sent. Zero
padding byte if there is tail room in skb.

Signed-of-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>

Signed-off-by: Jeff Garzik <jeff@garzik.org>