fbdev: offb: replace of_node_put with __free(device_node)

Replaced instance of of_node_put with __free(device_node)
to simplify code and protect against any memory leaks
due to future changes in the control flow.

Suggested-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: Abdulrasaq Lawani <abdulrasaqolawani@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>