iio: accel: adxl380: Introduce helper function for activity detection

iio: accel: adxl380: Introduce helper function for activity detection

Motion detection functionalities (such as activity and inactivity
detection) are only available when the chip is in a low-power mode; this
affects the available sampling frequency values.
In preparation for adding support for a new frequency value, introduce a
helper function that checks whether activity/inactivity detection is
currently enabled; this function will be reused in a future commit to
determine what frequency values are available at any given time.
No functional changes.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>