pciehp: Add missing memory barrier

Fix the possible race condition between pcie_isr() and pciehp_write_cmd()
because of the lack of memory barrier.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>