media: vivid: Fix V4L2_FIELD_ALTERNATE new frame check

The vivid driver will overlay stream time on generated frames. Though,
in interlacing mode V4L2_FIELD_ALTERNATE, each field is separate and
must have the same time to ensure proper render. Though, this time was
only updated every 2 frames as the code was checking against the wrong
counter (frame counter rather then field counter).

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>