[media] cx24120: Fix disecq_send_burst command

Previous version of this driver had a memset before every call
to cmd. This meant the default value of cmd.arg[1] was zero unless
burst is set. Make sure it remains zero.
Also fe_sec_mini_cmd_t is an enum, so test against it.

Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>