V4L/DVB (5866): ivtv: fix DMA timeout when capturing VBI + another stream

The VBI DMA is handled in a special way and is marked with a bit.
However, that bit was set at the wrong time and could be cleared
by mistake if a PCM (or other) DMA request would arrive before the
VBI DMA was completed. So on completion of the VBI DMA the driver
no longer knew that that DMA transfer was for VBI data. And this
in turn caused havoc with the card's DMA engine.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>