pciehp - fix wrong return value

This patch fixes the problem that trying to enable already enabled
slot disables the slot by returning the proper value from
pciehp_enable_slot()/pciehp_disable_slot().

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>