PCI: enhance physical slot debug information

Convert usages of pr_debug to dev_dbg and add physical slot name.

Note that we use dev_dbg on the struct pci_bus and still manually
print out the PCI slot number (instead of calling dev_dbg on a
pci_dev) because a struct pci_bus with empty physical slots will
not have any pci_devs.

Reviewed-by: Andrew Patterson <andrew.patterson@hp.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>