iio:dac:ad3552r: Fix an IS_ERR() vs NULL check

The fwnode_get_named_child_node() function does not return error
pointers.  It returns NULL.  Update the check accordingly.

Fixes: 8f2b5482

 ("drivers:iio:dac: Add AD3552R driver support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220404114244.GA19201@kili
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>