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>

target/ppc/mmu_helper.c

@@ -100,7 +100,7 @@ static int pp_check(int key, int pp, int nx)
         case 0x1:
         case 0x2:
             access |= PAGE_WRITE;
-            /* No break here */
+            /* fall through */
         case 0x3:
             access |= PAGE_READ;
             break;
@@ -708,7 +708,7 @@ static int mmu40x_get_physical_address(CPUPPCState *env, mmu_ctx_t *ctx,
             if (pr != 0) {
                 goto check_perms;
             }
-            /* No break here */
+            /* fall through */
         case 0x3:
             /* All accesses granted */
             ctx->prot = PAGE_READ | PAGE_WRITE | PAGE_EXEC;
@@ -722,7 +722,7 @@ static int mmu40x_get_physical_address(CPUPPCState *env, mmu_ctx_t *ctx,
                 ret = -2;
                 break;
             }
-            /* No break here */
+            /* fall through */
         case 0x1:
         check_perms:
             /* Check from TLB entry */