usb: dwc3: gadget: dwc3_gadget_del_and_unmap_request() can be static

Fixes the following sparse warning:

drivers/usb/dwc3/gadget.c:169:6: warning:
 symbol 'dwc3_gadget_del_and_unmap_request' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>