V4L/DVB (12413): radio-si470x: separate common and usb code

This patch is a preceding work to add the i2c interface of si470x.
The si470x directory includes a common file and usb specific file and
header file.
The part unrelated with usb interface and i2c interface exists in
radio-si470x-common.c file, and The usb specific part exists in
radio-si470x-usb.c file.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
[tobias.lorenz@gmx.net: Small changes, due to new include "linux/smp_lock.h"]
Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>