[media] mb86a20s: some fixes at preBER logic

The logic that resets the device is wrong. It should be resetting
just the layer that got read. Also, stop is needed before updating
the counters.
While there, rename it, as we'll soon introduce a postBER logic
there.

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