hw/block/pflash_cfi02: Explicit switch fallthrough for ERASE commands

Previous to commit ddb6f2254, the DQ2 bit was incorrectly set
during PROGRAM command (0xA0). The commit reordered the switch
cases to only set the DQ2 bit for the ERASE commands using a
fallthrough, but did not explicit the fallthrough is intentional.

Mark the switch fallthrough with a comment interpretable by C
preprocessors and static analysis tools.

Reported-by: Coverity (CID 1403012)
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20190711130759.27720-1-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

hw/block/pflash_cfi02.c

@@ -367,6 +367,7 @@ static uint64_t pflash_read(void *opaque, hwaddr offset, unsigned int width)
     case 0x30: /* Sector Erase */
         /* Toggle bit 2 during erase, but not program. */
         toggle_dq2(pfl);
+        /* fall through */
     case 0xA0: /* Program */
         /* Toggle bit 6 */
         toggle_dq6(pfl);