fsl/usb: Increase TXFIFOTHRESH value for usb write in T4 Rev 2.0

fsl/usb: Increase TXFIFOTHRESH value for usb write in T4 Rev 2.0

Increase TXFIFOTHRES field value in TXFILLTUNING register of usb for T4 Rev 2.0.
This decreases data burst rate with which data packets are posted from the TX
latency FIFO to compensate for latencies in DDR pipeline during DMA.
This avoids Tx buffer underruns and leads to successful usb writes

Signed-off-by: Ramneek Mehresh <ramneek.mehresh@freescale.com>
Signed-off-by: Nikhil Badola <nikhil.badola@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>