iio: adc: ad4695: use IIO_DECLARE_DMA_BUFFER_WITH_TS

Use IIO_DECLARE_DMA_BUFFER_WITH_TS() to declare the buffer that gets
used with iio_push_to_buffers_with_ts(). This makes the code a bit
easier to read and understand.

Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250507-iio-introduce-iio_declare_buffer_with_ts-v6-3-4aee1b9f1b89@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>