usb: dwc3: gadget: use TRB type 6 for ISOC transfers

Type 6 should be used for the first transfer during an interval. This is
also what the reference driver is using. Type 7 seems to be for following
or additional transfers within the same interval.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/gadget.c

@@ -632,7 +632,7 @@ static struct dwc3_request *dwc3_prepare_trbs(struct dwc3_ep *dep,
 			break;
 
 		case USB_ENDPOINT_XFER_ISOC:
-			trb.trbctl = DWC3_TRBCTL_ISOCHRONOUS;
+			trb.trbctl = DWC3_TRBCTL_ISOCHRONOUS_FIRST;
 
 			/* IOC every DWC3_TRB_NUM / 4 so we can refill */
 			if (!(cur_slot % (DWC3_TRB_NUM / 4)))