x86: only scan the root bus in early PCI quirks

We found a situation on Linus' machine that the Nvidia timer quirk hit on
a Intel chipset system.  The problem is that the system has a fancy Nvidia
card with an own PCI bridge, and the early-quirks code looking for any
NVidia bridge triggered on it incorrectly.  This didn't lead a boot
failure by luck, but the timer routing code selecting the wrong timer
first and some ugly messages.  It might lead to real problems on other
systems.

I checked all the devices which are currently checked for by early_quirks
and it turns out they are all located in the root bus zero.

So change the early-quirks loop to only scan bus 0.  This incidently also
saves quite some unnecessary scanning work, because early_quirks doesn't
go through all the non root busses.

The graphics card is not on bus 0, so it is not matched anymore.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>