net: replace PPP_MTU with PPP_MRU

Change-Id: Ic9761ef9750d01d98ca6592fbbd62b1013ce465b
diff --git a/drivers/net/ppp/pppolac.c b/drivers/net/ppp/pppolac.c
index c94b850..a5d3d63 100644
--- a/drivers/net/ppp/pppolac.c
+++ b/drivers/net/ppp/pppolac.c
@@ -310,7 +310,7 @@
 	po->chan.hdrlen = 12;
 	po->chan.private = sk_udp;
 	po->chan.ops = &pppolac_channel_ops;
-	po->chan.mtu = PPP_MTU - 80;
+	po->chan.mtu = PPP_MRU - 80;
 	po->proto.lac.local = unaligned(&addr->local)->u32;
 	po->proto.lac.remote = unaligned(&addr->remote)->u32;
 	atomic_set(&po->proto.lac.sequencing, 1);
diff --git a/drivers/net/ppp/pppopns.c b/drivers/net/ppp/pppopns.c
index fb81984..6016d29 100644
--- a/drivers/net/ppp/pppopns.c
+++ b/drivers/net/ppp/pppopns.c
@@ -290,7 +290,7 @@
 	po->chan.hdrlen = 14;
 	po->chan.private = sk_raw;
 	po->chan.ops = &pppopns_channel_ops;
-	po->chan.mtu = PPP_MTU - 80;
+	po->chan.mtu = PPP_MRU - 80;
 	po->proto.pns.local = addr->local;
 	po->proto.pns.remote = addr->remote;
 	po->proto.pns.data_ready = sk_raw->sk_data_ready;