usb: dwc3: gadget: fix pending isoc handling

If xfernotready is received and there is no request in request_list then
REQUEST_PENDING flag must be set, so that next request in ep queue is executed.

In case of isoc transfer, if xfernotready is already elapsed and even first
request has not been queued to request_list, then issue END TRANSFER, so that
you can receive xfernotready again and can have notion of current microframe.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>