usbnet: fix oops in usbnet_start_xmit

This patch fixes the bug added in commit v3.1-rc7-1055-gf9b491e
SKB can be NULL at this point, at least for cdc-ncm.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>