Add 'fall through' comments to case statements without break

These comments are used by static code analysis tools and in code reviews
to avoid false warnings because of missing break statements.

The case statements handled here were reported by coverity.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hw/pcnet.c

@@ -1505,6 +1505,7 @@ static void pcnet_bcr_writew(PCNetState *s, uint32_t rap, uint32_t val)
 #ifdef PCNET_DEBUG
        printf("BCR_SWS=0x%04x\n", val);
 #endif
+        /* fall through */
     case BCR_LNKST:
     case BCR_LED1:
     case BCR_LED2:

json-lexer.c

@@ -301,6 +301,7 @@ static int json_lexer_feed_char(JSONLexer *lexer, char ch, bool flush)
         case JSON_KEYWORD:
         case JSON_STRING:
             lexer->emit(lexer, lexer->token, new_state, lexer->x, lexer->y);
+            /* fall through */
         case JSON_SKIP:
             QDECREF(lexer->token);
             lexer->token = qstring_new();

qemu-option.c

@@ -214,13 +214,17 @@ static int parse_option_size(const char *name, const char *value, uint64_t *ret)
         switch (*postfix) {
         case 'T':
             sizef *= 1024;
+            /* fall through */
         case 'G':
             sizef *= 1024;
+            /* fall through */
         case 'M':
             sizef *= 1024;
+            /* fall through */
         case 'K':
         case 'k':
             sizef *= 1024;
+            /* fall through */
         case 'b':
         case '\0':
             *ret = (uint64_t) sizef;