[media] mb86a20s: improve bit error count for BER

Do a better job on setting the bit error counters, in order to
have all layer measures to happen in a little less than one
second.

Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>