hp-wmi: fix handling of platform device

The driver will not quite work if someone unbinds the platform device
from the platform driver via sysfs (moreover it will bomb is the driver
built into the kernel as hp_wmi_bios_remove is marked as __exit and will
not be present in the kernel).

To fix it let's use platform_driver_probe() instead of
platform_driver_register(), which disables binding/unbinding via sysfs.
This also allows us to mark hp_wmi_bios_setup as __init and discard it
once module is initialized.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>