PCI: pciehp: Always protect pciehp_disable_slot() with hotplug mutex

When called from pciehp_sysfs_disable_slot(), the call to
pciehp_disable_slot() was not protected by the hotplug mutex.

Hold slot->hotplug_lock while calling pciehp_disable_slot().

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rajat Jain <rajatxjain@gmail.com>