PCI: Clean up quirk_io_region

Before every call of quirk_io_region(), pci_read_config_word() is called.
We can fold that call into quirk_io_region() to make code more readable.

[bhelgaas: changelog, fill bus_region directly rather than copying from res]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>