staging:iio:mxs-lradc: Use iio_push_to_buffers_with_timestamp()

Makes the code a bit shorter and less ugly.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>