PCI: Add sysfs max_link_speed/width, current_link_speed/width, etc

Expose PCIe bridges attributes such as secondary bus number, subordinate
bus number, max link speed and link width, current link speed and link
width via sysfs in /sys/bus/pci/devices/...

This information is available via lspci, but that requires root privilege.

Signed-off-by: Wong Vee Khee <vee.khee.wong@ni.com>
Signed-off-by: Hui Chun Ong <hui.chun.ong@ni.com>
[bhelgaas: changelog, return errors early to unindent usual case, return
errors with same style throughout]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>