V4L/DVB (10983): v4l2-common: add missing i2c_unregister_device.

If the i2c sub-device cannot be found, then we must unregister the i2c_client.
Otherwise this will prevent a possible probe for a different device on that same
address.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>