fsl_usb2_udc: Add a wmb before priming endpoint.

Add a wmb to fsl_queue_td before priming the endpoint. This ensures that the
modifications to the QH are seen by the hardware.

Added comment as suggested by Felipe Balbi.

Signed-off-by: Will Newton <will.newton@gmail.com>
Acked-by: Li Yang <leoli@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>