wacom: Fix comment for timeout duration

The comment mentioned 5 seconds, but we use 15 seconds.
wacom: Fix ~100 pixel offset when calibrating

The calibration gui code moved the targets to x,y, instead of
placing the centre of the target at x,y, leading to a 47 pixel
(half the target's size) offset in both directions (thus 67 pixels
deviation as Pythagorus would tell us).