usb: cdns3: ep0: fix the test mode set incorrectly

The 'tmode' is ctrl->wIndex, changing it as the real test
mode value for register assignment.

Cc: <stable@vger.kernel.org>
Fixes: 7733f6c3 ("usb: cdns3: Add Cadence USB3 DRD Driver")
Reviewed-by: Jun Li <jun.li@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>