ADXL362: Do not clear last bits of 16-bit inactivity time value

The inactivity time registers identified by `ADXL362_REG_TIME_INACT_L`
and `ADXL362_REG_TIME_INACT_H` accepts a 16-bit value. (8 in each).
Without this change the last 5 bits of the register value
will be cleared.

Clearing the last bits of the register value greatly reduces the maximum
inactivity time that can be set.

Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>