staging: iio: cdc: ad7746: Use explicit be24 handling.

Chance from fiddly local implementation of be24 to cpu endian conversion
by reading into a 3 byte buffer and using get_unaligned_be24()

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-4-jic23@kernel.org