iio: frequency: ad9523: Fix typo in ad9523_platform_data

Replace diff_{m1,m2} with div_{m1,m2} since they are dividers and not a
differential settings.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>