Input: elants_i2c - provide an attribute to show calibration count

Input: elants_i2c - provide an attribute to show calibration count

There is an non-touch case by non-calibration after update firmware.
Elan could know calibrate or not by calibration count.
The value of '0xffff' means we didn't calibrate after update firmware.
If calibrate success, it will plus one and change to '0x0000'.

Signed-off-by: Johnny Chuang <johnny.chuang@emc.com.tw>
Link: https://lore.kernel.org/r/1588754932-5902-1-git-send-email-johnny.chuang.emc@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>