face: allow GenericLinkService to override MTU

refs #5056

Change-Id: I8eb2dd732b1431e0e809deb53792a14be1698ff1
diff --git a/tests/daemon/face/lp-reliability.t.cpp b/tests/daemon/face/lp-reliability.t.cpp
index 9c14997..1880074 100644
--- a/tests/daemon/face/lp-reliability.t.cpp
+++ b/tests/daemon/face/lp-reliability.t.cpp
@@ -809,7 +809,7 @@
   // for Sequence and TxSequence, leaving 6 octets for piggybacking. Each Ack header is 12 octets,
   // so there's no room to piggyback any Ack in LpPacket.
 
-  transport->setMtu(Transport::MIN_MTU);
+  transport->setMtu(MIN_MTU);
 
   for (lp::Sequence i = 1000; i < 1100; i++) {
     reliability->m_ackQueue.push(i);