media: si470x-i2c: add missed operations in remove

The driver misses calling v4l2_ctrl_handler_free and
v4l2_device_unregister in remove like what is done in probe failure.
Add the calls to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>