Input: mainstone-wm97xx - fix condition in pen_up

The loop body was never executed, because the condition is
always false. Convert to for with more obvious condition.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>