cx24116: fix a buffer overflow when checking userspace params

cx24116: fix a buffer overflow when checking userspace params

commit 1fa2337a315a2448c5434f41e00d56b01a22283c upstream.

The maximum size for a DiSEqC command is 6, according to the
userspace API. However, the code allows to write up much more values:
	drivers/media/dvb-frontends/cx24116.c:983 cx24116_send_diseqc_msg() error: buffer overflow 'd->msg' 6 <= 23

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>