usb: isp1760: fix spin unlock in the error path of isp1760_udc_start

Commit a124820de5fd ("usb: isp1760: fix possible deadlock in
isp1760_udc_irq") replaced spin_{un,}lock with spin_{un,}lock_irq{save,restore}.
However it missed an error path resulting in the smatch warning as below:

drivers/usb/isp1760/isp1760-udc.c:1230 isp1760_udc_start() warn: inconsistent returns 'irqsave:flags'.
Locked on:   line 1207
Unlocked on: line 1199

This patch fixes the spin unlock in the error path in isp1760_udc_start
thereby removing the smatch warning mentioned above.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/isp1760/isp1760-udc.c

@@ -1203,7 +1203,7 @@ static int isp1760_udc_start(struct usb_gadget *gadget,
 
 	if (udc->driver) {
 		dev_err(udc->isp->dev, "UDC already has a gadget driver\n");
-		spin_unlock(&udc->lock);
+		spin_unlock_irqrestore(&udc->lock, flags);
 		return -EBUSY;
 	}