HID: hid-sensor-hub: Use mfd_add_hotplug_devices() helper

Use mfd_add_hotplug_devices() helper to register the subdevices.

Compile-only tested.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>