PCI: pciehp: Make pcie_wait_cmd() self-contained

pcie_wait_cmd() waits for the controller to finish a hotplug command.  Move
the associated logic (to determine whether waiting is required and whether
we're using interrupts or polling) from pcie_write_cmd() to
pcie_wait_cmd().

No functional change.

Tested-by: Rajat Jain <rajatxjain@gmail.com>	(IDT 807a controller)
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>