V4L/DVB (9980): em28xx: simplify analog logic

Now, just two routines are enough for analog: the first one configs the analog
part and register V4L2 devices, and the second one release analog devices.

After this patch, it will be easier to transform em28xx-video into an em28xx
extension, loaded only on analog devices.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>