media: vivid: set min width/height to a value > 0

The capture DV timings capabilities allowed for a minimum width and
height of 0. So passing a timings struct with 0 values is allowed
and will later cause a division by zero.

Ensure that the width and height must be >= 16 to avoid this.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by:  <syzbot+57c3d83d71187054d56f@syzkaller.appspotmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>