[media] mb86a20s: fix audio sub-channel check

As reported by Dan Carpenter <dan.carpenter@oracle.com>
FYI, there are new smatch warnings show up in:
	tree:   git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next

 master
	head:   da17d7bda957ae4697b6abc0793f74fb9b50b58f
	commit: 04fa725e [media] mb86a20s: Implement set_frontend cache logic
	New smatch warnings:
	drivers/media/dvb-frontends/mb86a20s.c:1897 mb86a20s_set_frontend() error: buffer overflow 'mb86a20s_subchannel' 8 <= 8
	04fa725e Mauro Carvalho Chehab 2013-03-04  1894  		if (c->isdbt_sb_subchannel > ARRAY_SIZE(mb86a20s_subchannel))
	04fa725e Mauro Carvalho Chehab 2013-03-04  1895  			c->isdbt_sb_subchannel = 0;
	04fa725e Mauro Carvalho Chehab 2013-03-04  1896
	04fa725e Mauro Carvalho Chehab 2013-03-04 @1897  		state->subchannel = mb86a20s_subchannel[c->isdbt_sb_subchannel];

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
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>