[POWERPC] Fix incorrect calculation of I/O window addresses

My patch "Cope with PCI host bridge I/O window not starting at 0"
introduced a bug in the calculation of the virtual addresses for the
I/O windows of PCI host bridges other than the first, because it
didn't account for the fact that hose->io_resource gets offset so that
it reflects the range of global I/O port numbers assigned to the
bridge.  This fixes it and simplifies get_bus_io_range() in the
process.

Signed-off-by: Paul Mackerras <paulus@samba.org>