usb: gadget: atmel_usba: always test udc->driver

Found using smatch: drivers/usb/gadget/atmel_usba_udc.c:1689 usba_udc_irq()
error: we previously assumed 'udc->driver' could be null (see line 1636)

Always test udc->driver before using its members.
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/atmel_usba_udc.c

@@ -1686,7 +1686,7 @@ static irqreturn_t usba_udc_irq(int irq, void *devid)
 		reset_all_endpoints(udc);
 
 		if (udc->gadget.speed != USB_SPEED_UNKNOWN
-				&& udc->driver->disconnect) {
+				&& udc->driver && udc->driver->disconnect) {
 			udc->gadget.speed = USB_SPEED_UNKNOWN;
 			spin_unlock(&udc->lock);
 			udc->driver->disconnect(&udc->gadget);