amd64-agp: run fallback when no bridges found, not when driver registration fails

I think the intent was that if no bridges matched agp_amd64_pci_table[],
we would fall back to checking for any bridge with the AGP capability.
But in the current code, we execute the fallback path only when
pci_register_driver() itself fails, which is unrelated to whether any
matching devices were found.

This patch counts the AGP bridges found in the probe() method and executes
the fallback path when none is found.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>