[media] mb86a20s: Use 'layer' instead of 'i' on all places

We're using the anonymous 'i' to indicate the layer number
on several places on the driver. That's not good, as some
cut-and-paste type of change might be doing the wrong thing.
So, call it as "layer" everywhere.

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>