[media] adv7604/adv7842: fix il_vbackporch typo and zero the struct

Both adv7604 and adv7842 had the same typo in the code that sets
the vertical backporch for the second interlaced field: it was
assigned to vbackporch instead of il_vbackporch.

In addition, the timings struct wasn't zeroed in the adv7842 driver,
leaving several fields to undefined values causing the timing match
function to fail.

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