iio: adc: ad7280a: use IIO_DIFF_EVENT_CODE macro helper

iio: adc: ad7280a: use IIO_DIFF_EVENT_CODE macro helper

The IIO_DIFF_EVENT_CODE macro helper was introduced to provide a more
specific alternative to the generic IIO_EVENT_CODE macro for handling
differential channels. This commit updates the code to use
IIO_DIFF_EVENT_CODE for better clarity and maintainability.

However, the current implementation incorrectly sets both chan1 and
chan2 to 0. To maintain compatibility and avoid breaking existing
user space applications, this behavior is preserved for now.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241028-iio-add-macro-for-even-identifier-for-differential-channels-v1-2-b452c90f7ea6@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>