PCI: Remove local variable usage in pci_bus_distribute_available_resources()

In pci_bus_distribute_available_resources(), use resource_size() rather
than the local available_io, etc.  No functional change intended; this just
makes the preceding patch smaller.

[bhelgaas: extracted from https://lore.kernel.org/r/PSXP216MB0438587C47CBEDF365B1EA27803C0@PSXP216MB0438.KORP216.PROD.OUTLOOK.COM

]
Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>