thermal: i.MX: Allow thermal probe to fail gracefully in case of bad calibration.

Without this fix, the thermal probe on i.MX6 might trigger a division
by zero exception later in the probe if the calibration does fail.

Note: This linux behavior (Division by zero in kernel) has been triggered
on a Qemu i.MX6 emulation where parameters in nvmem were not set. With this
fix the division by zero is not triggeed anymore as the thermal probe does
fail early.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>