usb: misc: usbtest: usbtest_do_ioctl may return positive integer

usb: misc: usbtest: usbtest_do_ioctl may return positive integer

For case 14 and case 21, their correct return value is the number
of bytes transferred, so it is a positive integer. But in usbtest_ioctl,
it takes non-zero as false return value for usbtest_do_ioctl, so
it will treat the correct test as wrong test, then the time on
tests will be the minus value.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Cc: stable <stable@vger.kernel.org>
Fixes: 18fc4ebdc705 ("usb: misc: usbtest: Remove timeval usage")
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>