iio: imx7d_adc: Introduce a definition for the input clock

Since the input clock is always 24MHz, there is no need for storing
this value into a variable.

Use a definition instead, which is more appropriate in this case.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>