usb: gadget: uvc: Fix error handling in uvc_queue_buffer()

The conversion to videobuf2 failed to check the return value of
vb2_qbuf(). Fix it.

Cc: stable@vger.kernel.org
Reported-by: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-By: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>