iio: accel: adxl355: Make timestamp 64-bit aligned using aligned_s64

iio: accel: adxl355: Make timestamp 64-bit aligned using aligned_s64

The IIO ABI requires 64-bit aligned timestamps. In this case insufficient
padding would have been added on architectures where an s64 is only 32-bit
aligned.  Use aligned_s64 to enforce the correct alignment.

Fixes: 327a0eaf19d5 ("iio: accel: adxl355: Add triggered buffer support")
Reported-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250413103443.2420727-5-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>