iio: adc: ad7944: Fix sign and use aligned_s64 for timestamp.

Whilst it doesn't actually make any difference because the code
that fills this field doesn't care, timestamps are all signed.
Use the new aligned_s64 instead of open coding alignment to avoid
confusing static analyzers and give slightly cleaner code.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20241215182912.481706-2-jic23@kernel.org


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