iio: adc: hx711: use struct with aligned_s64 timestamp

Use a struct with aligned s64_timestamp instead of a padded array for
the buffer used for iio_push_to_buffers_with_ts(). This makes it easier
to see the correctness of the size and alignment of the buffer.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250418-iio-prefer-aligned_s64-timestamp-v1-3-4c6080710516@baylibre.com


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