fbdev: ssd1307fb: potential ERR_PTR dereference

The error handling got shifted down a few lines from where it was
supposed to be for some reason.

Fixes: a14a7ba8cb0f ('fbdev: ssd1307fb: add backlight controls for setting the contrast')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>