To properly detect frame loss the driver must keep
track of a frame_count.

Furthermore, field_count use was erroneous because
in progressive format this must be incremented twice.
Signed-off-by: NJavier Martin <javier.martin@vista-silicon.com>
Signed-off-by: NGuennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>