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.