usb: dwc2: gadget: prevent new request submission during suspend

If usb controller is in partial power down, any write to registers may
cause unpredictable behavior.
Thus, prevent any new request submission once controller is in partial
power down.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>