dm: pci: Use the correct hose when configuring devices

Only the PCI controller has access to the PCI region information. Make sure
to use the controller (rather than any attached bridges) when configuring
devices.

This corrects a failure to scan and configure devices when driver model is
enabled for PCI.

Also add a comment to explain the problem.

Signed-off-by: Simon Glass <sjg@chromium.org>