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>