[media] mb86a20s: better name temp vars at mb86a20s_layer_bitrate()

Using 'i' for the guard interval temporary var is a bad idea, as
'i' is generally used by "anonymous" indexes.
Let's rename modulation, fec and guard interval temp vars with
a meaningful name, as that makes easier to understand the code
and avoids cut-and-paste types of error.

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>