[media] v4l2-ioctl: zero the v4l2_bt_timings reserved field

This field should be zeroed. Both for s_dv_timings and for enum_dv_timings.
In fact, for the latter anything after the pad field can be zeroed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>