iio: adc: ad7944: use spi_optimize_message()

This modifies the ad7944 driver to use spi_optimize_message() to reduce
CPU usage and increase the max sample rate by avoiding repeating
validation of the spi message on each transfer.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240328-ad7944-spi-optimize-message-v2-1-a142b2576379@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>