V4L/DVB (11673): v4l2-device: unregister i2c_clients when unregistering the v4l2_device.

Until now I relied on i2c_del_adapter to unregister the i2c_clients for
me, however, if the i2c bus is a platform bus then it is never deleted.

So instead I need to unregister i2c clients when unregistering the
v4l2_device.

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