pciehp: Fix command write

Current implementation of pciehp_write_cmd() always enables command
completed interrupt. But pciehp_write_cmd() is also used for clearing
command completed interrupt enable bit. In this case, we must not set
the command completed interrupt enable bit. To fix this bug, this
patch add the check to see if caller wants to change command complete
interrupt enable bit.

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>