[POWERPC] Fix sys_pciconfig_iobase bus matching

A stupid bug has been plaguing the sys_pciconfig_iobase on ppc64. It wasn't
noticed until recently as it seems to not affect G5s but it's been causing
problems running X servers on some other machines recently. The bus number
matching was bogus.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>