iio: temperature: maxim_thermocouple: use DMA-safe buffer for spi_read()

Replace using stack-allocated buffers with a DMA-safe buffer for use
with spi_read(). This allows the driver to be safely used with
DMA-enabled SPI controllers.

The buffer array is also converted to a struct with a union to make the
usage of the memory in the buffer more clear and ensure proper alignment.

Fixes: 1f25ca11 ("iio: temperature: add support for Maxim thermocouple chips")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250721-iio-use-more-iio_declare_buffer_with_ts-3-v2-1-0c68d41ccf6c@baylibre.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>