V4L/DVB (12236): em28xx: stop abusing of board->decoder for sensor information

Instead of using em28xx board decoder field for storing sensor information,
let's use instead a separate field for it.

Also, as sensors are currently autodetected, there's no need of having
it at the boards description. So, move it to the main em28xx struct.

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