iio: adis. Drop adis_burst struct

As there are no users anymore of this structure, it can be safely
removed.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200917155223.218500-5-nuno.sa@analog.com