Input: synaptics - properly initialize slots for semi-MT

Semi-MT devices are pointers too, so let's tell that to
input_mt_init_slots(), as well as let it set up the devices as semi-MT,
instead of us doing it manually.

Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>