usb-bus: use snprintf

Avoid this warning from OpenBSD linker:
  LINK  i386-softmmu/qemu
../usb-bus.o(.text+0x27c): In function `usb_get_fw_dev_path':
/src/qemu/hw/usb-bus.c:294: warning: sprintf() is often misused,
please use snprintf()

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb-bus.c

@@ -298,20 +298,22 @@ static char *usb_get_fw_dev_path(DeviceState *qdev)
 {
     USBDevice *dev = DO_UPCAST(USBDevice, qdev, qdev);
     char *fw_path, *in;
-    int pos = 0;
+    ssize_t pos = 0, fw_len;
     long nr;
 
-    fw_path = qemu_malloc(32 + strlen(dev->port->path) * 6);
+    fw_len = 32 + strlen(dev->port->path) * 6;
+    fw_path = qemu_malloc(fw_len);
     in = dev->port->path;
-    while (true) {
+    while (fw_len - pos > 0) {
         nr = strtol(in, &in, 10);
         if (in[0] == '.') {
             /* some hub between root port and device */
-            pos += sprintf(fw_path + pos, "hub@%ld/", nr);
+            pos += snprintf(fw_path + pos, fw_len - pos, "hub@%ld/", nr);
             in++;
         } else {
             /* the device itself */
-            pos += sprintf(fw_path + pos, "%s@%ld", qdev_fw_name(qdev), nr);
+            pos += snprintf(fw_path + pos, fw_len - pos, "%s@%ld",
+                            qdev_fw_name(qdev), nr);
             break;
         }
     }