hw/block/pflash_cfi02: Rewrite a fall through comment

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

  hw/block/pflash_cfi02.c: In function ‘pflash_write’:
  hw/block/pflash_cfi02.c:574:16: error: this statement may fall through [-Werror=implicit-fallthrough=]
    574 |             if (boff == 0x55 && cmd == 0x98) {
        |                ^
  hw/block/pflash_cfi02.c:581:9: note: here
    581 |         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>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20190719131425.10835-4-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

hw/block/pflash_cfi02.c

@@ -577,7 +577,7 @@ static void pflash_write(void *opaque, hwaddr offset, uint64_t value,
                 pfl->cmd = 0x98;
                 return;
             }
-            /* No break here */
+            /* fall through */
         default:
             DPRINTF("%s: invalid write for command %02x\n",
                     __func__, pfl->cmd);