Input: wm9712 - fix return code for wrong sample

Instead of interpreting a wrong measurement as pen up, we should
try to read again.

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>