usb: misc: usbtest: remove redundant check on retval < 0

The check for retval being less than zero is always true since
retval equal to -EPIPE at that point.  Replace the existing
conditional with just return retval.

Detected with CoverityScan, CID#114349 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>