PCI: Move pci_read_bridge_windows() below individual window accessors

Move pci_read_bridge_windows() below the functions that read the I/O,
memory, and prefetchable memory windows, so pci_read_bridge_windows() can
use them in the future.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>