usb: dwc3: gadget: fix crash in dwc3_gadget_giveback()

usb: dwc3: gadget: fix crash in dwc3_gadget_giveback()

If the ep0 stalls or request are dequeued when gagdet is stopped,
the request dma may not be mapped yet and dwc3_flush_cache() may be
called with a NULL pointer.

Check req->request.dma before calling dwc3_flush_cache() and later
the usb_gadget_unmap_request() functions since it means that
usb_gadget_map_request() hasn't been called yet.

Fixes: fd15b58c1a9 ("dwc3: flush cache only if there is a buffer attached to a request")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20240528-topic-sm8x50-dwc3-gadget-crash-fix-v1-1-58434ab4b3d3@linaro.org
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>