V4L/DVB (7470): CX24123: preparing support for CX24113 tuner

To support a new device based on CX24123 (using the CX24113-tuner) the following was done:

- added two parameters to de-select the internal PLL-driver (for CX24108) and a AGC-function callback.
- added a virtual i2c-adapter which allow simple access behind the i2c-gate
- cleanup up some code

Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>