[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>

drivers/media/dvb-frontends/mb86a20s.c

@@ -610,7 +610,7 @@ static void mb86a20s_layer_bitrate(struct dvb_frontend *fe, u32 layer,
 	       __func__, 'A' + layer, segment * isdbt_rate[m][f][i]/1000,
 		rate, rate);
 
-	state->estimated_rate[i] = rate;
+	state->estimated_rate[layer] = rate;
 }