m68k: Handle 68040 bus faults

Fix 68040 bus fault handling, so the standard kernel exception handling
can be used for i/o probing.

Contrary to normal access faults there is nothing to fix, but at least
we have to disable writebacks to avoid recursive faults.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>