iio: backend: use __free(fwnode_handle) for automatic cleanup

iio: backend: use __free(fwnode_handle) for automatic cleanup

Convert __devm_iio_backend_fwnode_get() to use the __free(fwnode_handle)
cleanup attribute for the fwnode_back variable, removing the need for
manual fwnode_handle_put() calls. Move the declaration closer to its
first use, narrowing its scope.

No functional change.

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