hw/ppc/spapr.c: Extract MMU mode error reporting into a function

A following patch will make use of it.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20210505001130.3999968-2-farosas@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>