[media] dvb-core: prevent some corruption the legacy ioctl

Quite a few of the ->diseqc_send_master_cmd() implementations don't
check cmd->msg_len so it can lead to memory corruption.

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