V4L/DVB (6049): ivtv: fix VBI reinsertion decoding

- Invalid VBI packets should result in an empty VBI frame, not
  in an zero-sized frame that causes the reader to incorrectly
  return a 0 (EOF) value.
- PIO completion should not reset the sg_pending_size field.
- The DMA offset detection code should be ignored for PIO transfers:
  it somehow messes up the data on the card and is not needed anyway
  for PIO.

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