fbdev: sh_mobile_lcdcfb: fix notifier callback return codes

Notifier callbacks have to return either one of NOTIFY_* codes or a negative
errno, converted to a suitable value by the notifier_from_errno() inline, if
the notifier chain shall not be continued.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>