agp: fix SIS 5591/5592 wrong PCI id

The correct id is the id of the main host (5591) not
the id of the PCI-to-PCI bridge AGP (0001).
Output from "lspci -nv" shows that only the former
has AGP capabilities flag set:

00:00.0 0600: 1039:5591 (rev 02)
        Flags: bus master, medium devsel, latency 64
        Memory at ec000000 (32-bit, non-prefetchable) [size=32M]
        Capabilities: [c0] AGP version 1.0

00:02.0 0604: 1039:0001 (prog-if 00 [Normal decode])
        Flags: bus master, fast devsel, latency 0
        Bus: primary=00, secondary=01, subordinate=01, sec-latency=0
        I/O behind bridge: 0000c000-0000cfff
        Memory behind bridge: eb500000-eb5fffff
        Prefetchable memory behind bridge: eb300000-eb3fffff

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Dave Airlie <airlied@redhat.com>