DVD_INVALIDATE_AGID is a 'no payload' operation, so don't set

DVD_INVALIDATE_AGID is a 'no payload' operation, so don't set
xs->datalen and reset xs->data to NULL. This prevents a spurious
attempt to setup (dma map, etc) and possibly attempt data tranfser.

In line with what Linux and FreeBSD do as far as I can tell.

Reduces the delay before my DVD starts playing its movie.

Idea to also set xs->data to NULL from matthew@.

"No objection" matthew@