usb: renesas_usbhs: add DMAEngine ID specification note

renesas_usbhs DMAEngine uses D0FIFO/D1FIFO,
but the data transfer direction should be fixed for keeping consistency.
This patch explain about it on renesas_usbhs.h
Signed-off-by: NKuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

include/linux/usb/renesas_usbhs.h

@@ -132,6 +132,14 @@ struct renesas_usbhs_driver_param {
 	 * option:
 	 *
 	 * dma id for dmaengine
+	 * The data transfer direction on D0FIFO/D1FIFO should be
+	 * fixed for keeping consistency.
+	 * So, the platform id settings will be..
+	 *	.d0_tx_id = xx_TX,
+	 *	.d1_rx_id = xx_RX,
+	 * or
+	 *	.d1_tx_id = xx_TX,
+	 *	.d0_rx_id = xx_RX,
 	 */
 	int d0_tx_id;
 	int d0_rx_id;