[AGPGART] VIA PT880 Ultra support.

This patch enables agpgart on a Via "PT880 Ultra" based motherboard
(Asus P4V800D-X). The PCI ID of the PT880 Ultra is 0x0308 instead of
0x0258 of the PT880.

The patched via-agp passes testgart.

Signed-off-by: Magnus Kessler <Magnus.Kessler@gmx.net>
Signed-off-by: Dave Jones <davej@redhat.com>