xfree86: common: fix not-found check in xf86platformAddDevice()

Safer (and easier to understand) if we look at the result pointer
instead of the counter for testing whether device wasn't found.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

hw/xfree86/common/xf86platformBus.c

@@ -685,7 +685,7 @@ xf86platformAddDevice(const char *driver_name, int index)
         }
     }
 
-    if (i == xf86NumDrivers) {
+    if (!drvp) {
         ErrorF("can't find driver %s for hotplugged device\n", driver_name);
         return -1;
     }