classmate-laptop: add missing input_sync call

Add missing input_sync call in cmpc_keys_handler function.

Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>