V4L/DVB (11936): Fix v4l2-device usage of i2c_unregister_device()

Fix v4l2-device usage of i2c_unregister_device() and handle the case of
CONFIG_I2C=m & CONFIG_MEDIA_VIDEO=y.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>