Kernel: Use a public member for NetworkAdapter on_receive

Author: https://github.com/deoxxa
Commit: 498f8c01a2
Pull-request: https://github.com/SerenityOS/serenity/pull/499
Reviewed-by: https://github.com/awesomekling

Kernel/Net/NetworkAdapter.cpp

@@ -88,8 +88,8 @@ void NetworkAdapter::did_receive(const u8* data, int length)
     m_packets_in++;
     m_bytes_in += length;
     m_packet_queue.append(KBuffer::copy(data, length));
-    if (m_on_receive)
-        m_on_receive();
+    if (on_receive)
+        on_receive();
 }
 
 Optional<KBuffer> NetworkAdapter::dequeue_packet()