iio: sx9310: Fixes various memory handling

Makes use __aligned(8) to ensure that the timestamp is correctly aligned
when we call io_push_to_buffers_with_timestamp().
Also makes use of sizeof() for regmap_bulk_read instead of static value.

Signed-off-by: Daniel Campello <campello@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>