usb: dwc3: gadget: Add the suspend state checking when stopping gadget

It will be crash to stop gadget when the dwc3 device had been into suspend
state, thus we need to check if the dwc3 device had been into suspend state
when UDC try to stop gadget.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>