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

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