touchscreen_calibration: relax the threshold for untouched deltas

Relaxed the threshold of untouched deltas values a little bit so that
it would not generate false negatives unnecessarily.

BUG=chrome-os-partner:43044
TEST=manually tested.

Change-Id: I044d0d2539123505c7fe7ec82890c785f0079bd2
Reviewed-on: https://chromium-review.googlesource.com/290227
Reviewed-by: Chung-yih Wang <cywang@chromium.org>
Commit-Queue: Shyh-In Hwang <josephsih@chromium.org>
Tested-by: Shyh-In Hwang <josephsih@chromium.org>
1 file changed