Clear the ts event buffer at appropriate points in ts_calibrate

Clear the ts event buffer at appropriate points in ts_calibrate

Fixes the issue where an accidental double-tap at one coordinate results in
having to restart the calibration, since the second tap gets read as the next
coordinate for the calibration.  Patch courtesy MontaVista Software, Inc.

Signed-off-by: Chris Larson <clarson@kergoth.com>