Input: hp-wmi - switch to using sparse keymap library

Instead of implementing its own version of keymap hanlding switch over
to using sparse keymap library.

Also make sure that we install notify handler only after we allocated
input device and that we remove notify handler before unregistering
input device.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>