usb: dwc3: gadget: Refactor loop to avoid NULL endpoints

usb: dwc3: gadget: Refactor loop to avoid NULL endpoints

Prepare the gadget driver to handle the reserved endpoints that will be
not allocated at the initialisation time.

While at it, add a warning where the NULL endpoint should never happen.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Ferry Toth <fntoth@gmail.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20250212193116.2487289-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>