usb: gadget: uvc: fix possible lockup in uvc gadget

If the pending buffers in the queue could not be pushed to the udc
endpoint we have to cancel the uvc_queue. Otherwise the gadget will get
stuck on this error. This patch calls uvc_queue_cancel if usb_ep_queue
failed.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>