[media] mb86a20s: calculate statistics at .read_status()

Instead of providing separate callbacks to read the several FE
stats properties, the better seems to use just one method that will:
    - Read lock status;
    - Read signal strength;
    - if locked, get TMCC data;
    - if locked, get DVB statistics.
As the DVB frontend thread will call this read_status callback
on every 3 seconds, and userspace can even call it earlier,
all stats data and layers layout will be updated together if
available, with is a good thing.
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>