media: vivid: set field to NONE for touch

The v4l2_buffer's 'field' value was never initialized in vivid for the
touch capture device, causing v4l2-compliance errors.

Set it to NONE.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>