PCI: pciehp: Drop enable/disable lock

Previously slot enablement and disablement could happen concurrently.
But now it's under the exclusive control of the IRQ thread, rendering
the locking obsolete.  Drop it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>