PCI/portdrv: Don't create hotplug slots unless port supports hotplug

Commit 2dcfaf85

 mistakenly dropped the "flags & PCI_EXP_FLAGS_SLOT" test,
so now we create hotplug slots even for PCIe port devices that don't
support hotplug.  This patch fixes this problem.

[bhelgaas: changelog]
Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Reviewed-by: Jiang Liu <jiang.liu@huawei.com>