V4L/DVB (12415): radio-si470x: add disconnect check function

The si470x_disconnect_check is function to check disconnect state of
radio in common file. The function is implemented in each interface
file.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>