[media] mb86a20s: Use a macro for the number of layers

Instead of using the magic number "3", use NUM_LAYERS macro
on all places that are related to the ISDB-T layers.
This makes the source code a little more readable.
No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>