[media] mb86a20s: Fix estimate_rate setting

As reported by Dan Carpenter <dan.carpenter@oracle.com>:

	Smatch warnings:
	drivers/media/dvb-frontends/mb86a20s.c:644 mb86a20s_layer_bitrate() error: buffer overflow 'state->estimated_rate' 3 <= 3

What happens there is that estimate_rate index should be the layer
number, and not the guard interval.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>