[media] s5h1420: fix a buffer overflow when checking userspace params

The maximum size for a DiSEqC command is 6, according to the
userspace API. However, the code allows to write up to 7 values:
	drivers/media/dvb-frontends/s5h1420.c:193 s5h1420_send_master_cmd() error: buffer overflow 'cmd->msg' 6 <= 7

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>