Input: elantech - add support for elantech fast command

Starting with v3 hardware, the firmware supports this shorter
elantech_send_cmd. Teach the driver to use it.

Signed-off-by: JJ Ding <jj_ding@emc.com.tw>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>