staging: iio: ad7746: Automatically swap values in readings/writings

Data to read or write was being handled with the swab16() macro instead
of using i2c_smbus_{read,write}_swapped.

Signed-off-by: Hernán Gonzalez <hernan@vanguardiasur.com.ar>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>