iio: adc: xilinx-xadc-core: use devm_kmemdup_array()

Convert to use devm_kmemdup_array() and while at it, use source size
instead of destination.

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>