media: lib/test_printf.c: Use helper function to unwind array of software_nodes

Use the software_node_unregister_nodes() helper function to unwind this
array in a cleaner way.

Acked-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>