PCI: pciehp: Drop hotplug_slot_ops wrappers

pciehp's ->enable_slot, ->disable_slot, ->get_attention_status and
->reset_slot callbacks are currently implemented by wrapper functions
that do nothing else but call down to a backend function.  The backends
are not called from anywhere else, so drop the wrappers and use the
backends directly as callbacks, thereby shaving off a few lines of
unnecessary code.

No functional change intended.

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