Input: resistive-adc-touch - fix division by zero error on z1 == 0

For proper pressure calculation we need at least x and z1 to be non
zero. Even worse, in case z1 we may run in to division by zero
error.

Fixes: 60b7db91

 ("Input: resistive-adc-touch - rework mapping of channels")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20211007095727.29579-1-o.rempel@pengutronix.de


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>