x86_64: off-by-two error in aperture.c

I'm using a custom BIOS to configure the northbridge GART at address
0x80000000, size 2G.  Linux complains:

"Aperture from northbridge cpu 0 beyond 4GB. Ignoring."

I think there's an off-by-two error in arch/x86_64/kernel/aperture.c:

AK: use correct types for i386

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>