radio-si470x: separate usb and i2c interface

Hi, all

I send the radio-si470x patches worked on http://linuxtv.org/hg/v4l-dvb.

I have a board with Silicon Labs si4709 chip using the i2c interface, but the radio-si470x is only support usb interface currently. I posted about separating usb and i2c interface in radio-si470x the past.
http://www.mail-archive.com/[email protected]/msg02483.html

Tobias informed me the base code for seperating at
http://linuxtv.org/hg/~tlorenz/v4l-dvb of Tobias repository in above mail, i based on it, but it cannot find now at Tobias repository.

The patch 1/2 is for separating common and usb code, and the patch 2/2 is for supporting si470x i2c interface.

Please review, thank you.