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>