drivers: sensor: st: lis2du12: fix missing power down before sw reset

According to the LIS2DU12 datasheet (p.30), the device must be
inpower-down mode before executing a software reset.
Entering power-down requires setting the ODR field to 0x000.
This commit adds the missing call to set ODR=0x000 before issuing the
reset. Without this step, the sensor does not recover after a controller
reset and only resumes operation after a full power cycle.

Signed-off-by: Jan Kablitz <jan.kablitz@8tronix.de>