media: v4l2-dv-timings.c: fix too strict blanking sanity checks

Sanity checks were added to verify the v4l2_bt_timings blanking fields
in order to avoid integer overflows when userspace passes weird values.

But that assumed that userspace would correctly fill in the front porch,
backporch and sync values, but sometimes all you know is the total
blanking, which is then assigned to just one of these fields.

And that can fail with these checks.

So instead set a maximum for the total horizontal and vertical
blanking and check that each field remains below that.

That is still sufficient to avoid integer overflows, but it also
allows for more flexibility in how userspace fills in these fields.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 4b6d66a4

 ("media: v4l2-dv-timings: add sanity checks for blanking values")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>