pciehp: cleanup pcie_poll_cmd

Cleanup pcie_poll_cmd(): check the slot status once before entering our
completion test loop and convert the loop to a simpler while() block.

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