iio: adc: at91-sama5d2: Fix buffer alignment in iio_push_to_buffers_with_timestamp()

iio: adc: at91-sama5d2: Fix buffer alignment in iio_push_to_buffers_with_timestamp()

[ Upstream commit 8f884758966259fa8c50c137ac6d4ce9bb7859db ]

To make code more readable, use a structure to express the channel
layout and ensure the timestamp is 8 byte aligned.

Found during an audit of all calls of this function.

Fixes: db4fc7863dcf ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210613152301.571002-2-jic23@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>