iio: imu: adis16400: move burst logic to ADIS lib

This change has been done separately, so that it's easier to visualize the
changed logic in the adis_scan_update() function.

Most of the function in this `adis16400_update_scan_mode()` that deals with
burst-mode will be re-used in the ADIS16480, but with different parameters.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>