[media] coda: Unregister v4l2 upon alloc_workqueue() error

If alloc_workqueue() fails, we should go to the 'err_v4l2_register' label, which
will unregister the v4l2 device.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>