sh_mobile_meram: Safely disable MERAM operation when not initialized

If the MERAM platform data is defined, but the MERAM has not been
properly initaliazed we need to safely fall back to non-MERAM operation.

Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>