[media] cx2341x: fix kernel oops

The v4l2_ctrl_config struct must be zeroed before passing it to
v4l2_ctrl_new_custom(). This was always wrong, but with the recent
v4l2-ctrls.c changes this is now much more likely to lead to a
kernel bug.

This is the only place where this struct wasn't initialized properly.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Pridvorov Andrey <ua0lnj@bk.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>