usb: dwc2: gadget: Add dwc2_gadget_start_next_request function

Replaced repeating code with function call.

Starts next request from ep queue.
If queue is empty and ep is isoc
-In case of OUT-EP unmasks OUTTKNEPDIS.

OUTTKNEPDIS is masked in it's handler, so we need to unmask it here
to be able to do resynchronization.

Tested-by: John Keeping <john@metanate.com>
Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>