usb: usbtest: avoid integer overflow in test_ctrl_queue()

Avoid overflowing context.count = param->sglen * param->iterations,
where both `sglen' and `iterations' are from userspace.

| test_ctrl_queue()
| usbtest_ioctl()

Keep -EOPNOTSUPP for error code.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>