PCI ASPM: fix possible null pointer dereference

Fix possible NULL dereference in pcie_aspm_exit_link_state(). This
patch also cleanup some code.

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>