Input: elan_i2c - support touchpads with two physical buttons

Elan touchpads on Asus ROG G752xx series laptops have 2 physical buttons.
Luckily we can query the touchpad to see if it is a clickpad variant and
adjust the behavior accordingly.

Signed-off-by: KT Liao <kt.liao@emc.com.tw>
Tested-by: Maxime Bellengé <maxime.bellenge@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>