iio: accel: adxl345: extend inactivity time for less than 1s

iio: accel: adxl345: extend inactivity time for less than 1s

Inactivity and free-fall events are essentially the same type of sensor
events. Therefore, inactivity detection (normally set for periods between 1
and 255 seconds) can be extended for shorter durations to support free-fall
detection.

For periods shorter than 1 second, the driver automatically configures the
threshold and duration using the free-fall register. For periods longer
than 1 second, it uses the inactivity threshold and duration using the
inactivity registers.

When using the free-fall register, the link bit is not set, which means
auto-sleep cannot be enabled if activity detection is also active.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250727210014.27766-6-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>