mips jazz: do not raise data bus exception when accessing invalid addresses

MIPS Jazz chipset doesn't seem to raise data bus exceptions on invalid accesses.
However, there is no easy way to prevent them. Creating a big memory region
for the whole address space doesn't prevent memory core to directly call
unassigned_mem_read/write which in turn call cpu->do_unassigned_access,
which (for MIPS CPU) raise an data bus exception.

This fixes a MIPS Jazz regression introduced in c658b94f6e8c206c59d02aa6fbac285b86b53d2c.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
