PCI/portdrv: Use is_pciehp instead of is_hotplug_bridge

The PCIe port driver erroneously creates a subdevice for hotplug on ACPI
slots which are handled by the ACPI hotplug driver.

Avoid by checking the is_pciehp flag instead of is_hotplug_bridge when
deciding whether to create a subdevice.  The latter encompasses ACPI slots
whereas the former doesn't.

The superfluous subdevice has no real negative impact, it occupies memory
and interrupt resources but otherwise just sits there waiting for
interrupts from the slot that are never signaled.

Fixes: f8415222 ("PCI: Use cached copy of PCI_EXP_SLTCAP_HPC bit")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org # v4.7+
Link: https://patch.msgid.link/40d5a5fe8d40595d505949c620a067fa110ee85e.1752390102.git.lukas@wunner.de