PCI ASPM: remove get_root_port_link

By having a pointer to the root port link, we can remove loops in
get_root_port_link() to search the root port link.

Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>