pciehp: remove needless command completed interrupt setting

Currently, pciehp driver enables command completed interrupt as follows.

(1) Don't enable at initialization.
(2) Enable command completed interrupt whenever pciehp issues a
    command, if the command doesn't attempt to disable the interrupt.
(3) Disable command completed interrupt at driver unloading.

Once we enable command completed interrupt, we don't need to re-enable
it for every command. So we can simplify above steps as follows:

(1) Enable command completed interrupt at initialization.
(2) No special sequence for command completed interrupt.
(3) Disable command completed interrupt at driver unloading.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>