net/devif: fix devif_poll loop logic

Previously, the devif_poll works in this flow:
devif_poll_xx -> bstop = callback -> continue if !bstop -> poll another

Now we split the polling and callback, so it should work like:
devif_poll_connections -> return true if callback need to be called
   -> bstop = callback -> loop if !bstop -> poll again

Conditions:
poll_connections == 0, d_len == 0, break, return 0
poll_connections == 1, d_len == 0, break, return 1 (other stop case,
                                                    don't callback)
poll_connections == 1, d_len > 0, callback == 1, break, return 1
poll_connections == 1, d_len > 0, callback == 0, loop

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

net/devif/devif_poll.c

@@ -964,8 +964,14 @@ int devif_poll(FAR struct net_driver_s *dev, devif_poll_callback_t callback)
                 }
             }
         }
+      else
+        {
+          /* Not stopped by devif_poll_callback, just stop and return bstop */
+
+          break;
+        }
     }
-  while (bstop);
+  while (!bstop);
 
   /* Device polling completed, release iob */