iio: magn: rm3100: Fix alignment of buffer in iio_push_to_buffers_with_timestamp()

Add __aligned(8) to ensure the buffer passed to
iio_push_to_buffers_with_timestamp() is suitable for the naturally
aligned timestamp that will be inserted.

Here an explicit structure is not used, because this buffer is used in
a non-trivial way for data repacking.

Fixes: 121354b2 ("iio: magnetometer: Add driver support for PNI RM3100")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Song Qiang <songqiang1304521@gmail.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210613152301.571002-6-jic23@kernel.org