Input: psmouse - expose drift duration for IBM trackpoints

IBM Trackpoints have a feature to compensate for drift by recalibrating
themselves periodically. By default, if for 0.5 seconds there is no change
in position, it's used as the new zero. This duration is too low. Often,
the calibration happens when the trackpoint is in fact being used.

IBM's Trackpoint Engineering Specifications show a configuration register
that allows changing this duration, rstdft1.

Expose it via sysfs among the other settings.

Signed-off-by: Mike Murdoch <main.haarp@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>