usb: dwc2: gadget: Fix issue in dwc2_gadget_start_isoc()

In case of requests queue is empty reset EP target_frame to
initial value.

This allow restarting ISOC traffic in case when function
driver queued requests with interruptions.

Tested-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>