PCI hotplug: add support for 5.0G link speed

Add support for PCI-E 5.0 GT/s in max_bus_speed and cur_bus_speed.

Reviewed-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>