[media] usbtv: discard redundant video fields

There are many dropped fields with some sources, leading to many
redundant fields without counterparts. When this redundant field
is odd, a new frame is pushed containing this odd field interleaved
with whatever was left in the buffer, causing video artifacts.

Do not push a new frame after processing every odd field, but do it
only after those which come after an even field.

Signed-off-by: Nikola Forró <nikola.forro@gmail.com>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>