usb: dwc3: avoid NULL access of usb_gadget_driver

we found crash in dwc3_disconnect_gadget(),
it is because dwc->gadget_driver become NULL before async access.
7dc0c55e

 ('USB: UDC core: Add udc_async_callbacks gadget op')
suggest a common way to avoid such kind of issue.

this change implment the callback in dwc3 and
change related functions which have callback to usb gadget driver.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Linyu Yuan <linyyuan@codeaurora.org>
Link: https://lore.kernel.org/r/20210629015118.7944-1-linyyuan@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>