Cleanup usbnet_probe() return value handling

usbnet_probe() handles a positive return value from the driver bind()
function as success, but will later only setup the status handler if the
return value was zero, leading to confusion. Patch adjusts this to accept
positive values as success in both checks.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>