[media] vivi: fix sequence counting

The sequence counting was not reset to 0 between each streaming run,
and it was increased only every other frame. This is incorrect behavior:
the confusion is with FIELD_ALTERNATE systems where each field is transmitted
separately and only when both fields have been received is the frame
sequence number increased.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>