pciehp - add missing locking

This patch fixes the problem that system will panic if multiple power
on/off operations are issued to the same slot in parallel. This
problem can be easily reproduced by commands below.

    # while true; do echo 1 > power; echo 0 > power; done &
    # while true; do echo 1 > power; echo 0 > power; done &

The cause is lack of locking for enable/disable operations. This patch
fixes this problem.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>