video: fbdev: offb: switch to using for_each_node_by_type

Instead of open-coding loops let's switch to a nice macro.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Yongji Xie <xyjxie@linux.vnet.ibm.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>