V4L/DVB (9218): si470x: removement of get/set input/audio

This patch removes the unnecessary get/set input/audio functions.

The reason is, that the V4L2 specification says, that if input or audio
cannot be switched anyway, the functions doesn't need to be implemented.

I've tested the new driver with all current radio programs in
Debian/testing and found no problems with that.

In my opinion, the driver is much cleaner by removing these unnecessary
functions.

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>