iio: adis16475: Drop adis_burst usage

Burst mode variables are now part of the `adis_data` struct. The driver
also has now to explicitly define the length of the burst buffer.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200917155223.218500-4-nuno.sa@analog.com