usb: dwc3: gadget: add missing spin_lock()

commit 8e74475b0e0a (usb: dwc3: gadget: use udc-core's
reset notifier) added support for the new UDC core's
reset notifier to dwc3 but while at it, it removed
a spin_lock() from dwc3_reset_gadget() which might
cause an unbalanced spin_unlock() further down the line

Fixes: 8e74475b0e0a (usb: dwc3: gadget: use udc-core's reset notifier)
Cc: <stable@vger.kernel.org> # v3.19
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/gadget.c

@@ -2045,6 +2045,7 @@ static void dwc3_resume_gadget(struct dwc3 *dwc)
 	if (dwc->gadget_driver && dwc->gadget_driver->resume) {
 		spin_unlock(&dwc->lock);
 		dwc->gadget_driver->resume(&dwc->gadget);
+		spin_lock(&dwc->lock);
 	}
 }