toshiba-acpi - switch to using sparse keymap

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

Also, install notify handler only after we allocated input device,
otherwise we may risk getting event too early and crash. Similarly,
notify handler should be removed before we unregister input device.

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