net: usbnet: fix SG initialisation

Commit 60e453a9 ("USBNET: fix handling padding packet")
added an extra SG entry in case padding is necessary, but
failed to update the initialisation of the list. This can
cause list traversal to fall off the end of the list,
resulting in an oops.

Fixes: 60e453a9 ("USBNET: fix handling padding packet")
Reported-by: Thomas Kear <thomas@kear.co.nz>
Cc: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Tested-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>