iio: potentiostat: lmp91000: 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 structure is not used, because this buffer is also used
elsewhere in the driver.

Fixes: 67e17300 ("iio: potentiostat: add LMP91000 support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20210501171352.512953-8-jic23@kernel.org