sh: disabled cache handling fix.

Add code to handle the cache disabled case. Fixes breakage introduced by
37443ef3 ("sh: Migrate SH-4 cacheflush
ops to function pointers."). Without this patch configuring caches off
with CONFIG_CACHE_OFF=y makes kfr2r09 and migo-r lock up in fbdev
deferred io or early user space.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>