[AGPGART] Fix PCI-posting flush typo.

Unfortunately there was a typo in one of the patches I sent,
(The one now committed to the agpgart tree).
It may cause a bus error on i810 type hardware.

Signed-off-by: Thomas Hellstrom <thomas@tungstengraphics.com>
Signed-off-by: Dave Jones <davej@redhat.com>