iio: frequency: ad9523: add dev variable

iio: frequency: ad9523: add dev variable

Introduce a local struct device variable in ad9523_probe() to simplify
subsequent conversions and improve code readability.

Split pdata declaration and assignment since the result is validated
immediately after.

No functional change.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>