hw/ipmi: Rewrite a fall through comment

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

  hw/ipmi/ipmi_bmc_extern.c: In function ‘addchar’:
  hw/ipmi/ipmi_bmc_extern.c:178:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
    178 |         ch |= 0x10;
        |         ~~~^~~~~~~
  hw/ipmi/ipmi_bmc_extern.c:181:5: note: here
    181 |     default:
        |     ^~~~~~~
  cc1: all warnings being treated as errors
  make: *** [rules.mak:69: hw/ipmi/ipmi_bmc_extern.o] Error 1

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: Corey Minyard <cminyard@mvista.com>
Message-Id: <20190719131425.10835-5-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/ipmi/ipmi_bmc_extern.c

@@ -177,8 +177,7 @@ static void addchar(IPMIBmcExtern *ibe, unsigned char ch)
         ibe->outbuf[ibe->outlen] = VM_ESCAPE_CHAR;
         ibe->outlen++;
         ch |= 0x10;
-        /* No break */
-
+        /* fall through */
     default:
         ibe->outbuf[ibe->outlen] = ch;
         ibe->outlen++;