iio: adc: xilinx-xadc: Drop duplicate NULL check in xadc_parse_dt()

The fwnode_for_each_child_node() is NULL-aware, no need to check
its parameters outside. Drop duplicate NULL check in xadc_parse_dt().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220531141118.64540-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>