iio: imu: adis16xxx: use helper to access iio core debugfs dir

The IIO core provides a iio_get_debugfs_dentry() helper.
It seems that the ADIS IMU drivers access that field directly.

This change converts them to use iio_get_debugfs_dentry() instead.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>