[media] em28xx: move analog-specific init to em28xx-video

There are several init code inside em28xx-cards that are actually
part of analog initialization. Move the code to em28x-video, in
order to remove part of the mess.

In thesis, no functional changes so far.

Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>