topstar-laptop - switch to using sparse keymap library

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

Acked-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>