Input: elan_i2c - change the hover event from MT to ST

The firmware only reports hover condition while the very first contact is
approaching the surface; the hover is not reported for the subsequent
contacts. Therefore we should not be using ABS_MT_DISTANCE to report hover
but rather its single-touch counterpart ABS_DISTANCE.

Signed-off-by: Duson Lin <dusonlin@emc.com.tw>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>