[usb netdev] asix: fix regression

51bf2976 caused a regression in the asix
usbnet driver. usb_control_msg returns the number of bytes read on
success, not 0. Tested with NETGEAR FA120.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>