Fix compiler warnings (#6023)

components/net/netdev/src/netdev.c

@@ -1066,7 +1066,8 @@ int netdev_cmd_ping(char* target_name, char *netdev_name, rt_uint32_t times, rt_
 
     struct netdev *netdev = RT_NULL;
     struct netdev_ping_resp ping_resp;
-    int index, ret = 0;
+    rt_uint32_t index;
+    int ret = 0;
 
     if (size == 0)
     {

components/net/sal/src/sal_socket.c

@@ -376,7 +376,8 @@ static void sal_unlock(void)
  */
 int sal_netdev_cleanup(struct netdev *netdev)
 {
-    int idx = 0, find_dev;
+    uint32_t idx = 0;
+    int find_dev;
 
     do
     {