chardev: Use goto/label instead of do/break/while(0)

Use of a do/while(0) control flow in order to permit an early break
is an unusual paradigm, and triggers a false positive with a planned
future syntax check against 'while (0);'.  Rewrite the code to use a
goto instead.  This patch temporarily keeps an extra level of
indentation to highlight the change; the next patch cleans it up.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20171201232433.25193-4-eblake@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

chardev/char-serial.c

@@ -64,9 +64,14 @@ static void tty_serial_init(int fd, int speed,
 #endif
     tcgetattr(fd, &tty);
 
-#define check_speed(val) if (speed <= val) { spd = B##val; break; }
+#define check_speed(val) \
+    if (speed <= val) {  \
+        spd = B##val;    \
+        goto done;       \
+    }
+
     speed = speed * 10 / 11;
-    do {
+    {
         check_speed(50);
         check_speed(75);
         check_speed(110);
@@ -125,8 +130,10 @@ static void tty_serial_init(int fd, int speed,
         check_speed(4000000);
 #endif
         spd = B115200;
-    } while (0);
+    }
 
+#undef check_speed
+ done:
     cfsetispeed(&tty, spd);
     cfsetospeed(&tty, spd);