usb: dwc3: gadget: Fix starting microframe for ISOC

The gadget wants to set the starting microframe for the first ISOC TRB
to 4 microframes in the future, but it does so by multiplying the
dep->interval. This only works if dep->interval = 1. For other intervals
it will put it 4 *intervals* in the future which may be way too much.

Fix so that it always adds just one interval or at least 4 microframes.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>