iio: adc: ad7266: Fix potential timestamp alignment issue.

On architectures where an s64 is only 32-bit aligned insufficient padding
would be left between the earlier elements and the timestamp. Use
aligned_s64 to enforce the correct placement and ensure the storage is
large enough.

Fixes: 54e018da ("iio:ad7266: Mark transfer buffer as __be16") # aligned_s64 is much newer.
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/20250413103443.2420727-2-jic23@kernel.org


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>