media: radio: Critical v4l2 registration bugfix for si470x over i2c

Added the call to v4l2_device_register() required to add a new radio device.
Without this patch, it is impossible for the driver to load. This does not
affect USB devices.

Signed-off-by: Douglas Fischer <fischerdouglasc@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>