target/ppc: Rewrite a fall through comment

target/ppc: Rewrite a fall through comment

GCC9 is confused by this comment when building with CFLAG
-Wimplicit-fallthrough=2:

  target/ppc/mmu_helper.c: In function ‘dump_mmu’:
  target/ppc/mmu_helper.c:1349:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
   1349 |         if (ppc64_v3_radix(env_archcpu(env))) {
        |            ^
  target/ppc/mmu_helper.c:1356:5: note: here
   1356 |     default:
        |     ^~~~~~~
  cc1: all warnings being treated as errors

Rewrite the comment using 'fall through' which is recognized by
GCC and static analyzers.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190719131425.10835-6-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>