of: Make of_fwnode_handle() safer

On the expense of a little bit more complexity in the of_fwnode_handle()
macro, make the macro result in NULL in case its argument is NULL while
still referencing it only once.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Rob Herring <robh@kernel.org>