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

commit ebe864a6cb8e087ede047fa1fa6b6d06fcb9a9e4 upstream.

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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>