usb-bus: fix no params

After commit 702f3e0fb5, the params is
nerver NULL. It should check *params instead of params to determine
whether the params is empty.

Signed-off-by: TeLeMan <geleman@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

hw/usb-bus.c

@@ -299,7 +299,7 @@ USBDevice *usbdevice_create(const char *cmdline)
     }
 
     if (!usb->usbdevice_init) {
-        if (params) {
+        if (*params) {
             error_report("usbdevice %s accepts no params", driver);
             return NULL;
         }