Input: wm9712 - fix wrong pen up readings

Often a reading can be wrong. This patch assures that this is really a
pen up event and not a false reading.

Based on wm9712: pen up by Teresa Gámez and Christian Hemp.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>