Fix an integer comparison always false warning in the PCI Calgary 64 driver.

A u8 is being compared to something that's 512 by default, resulting in the
following warning:

arch/x86/kernel/pci-calgary_64.c:1285: warning: comparison is always false due to limited range of data type

This was introduced by patch b34e90b8.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>