fbdev: ssd1307fb: set default height if not found in DT node

this patch sets the default height if its not found in DT.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>