set mtu to 1300 because mtu detection is absolutely not reliable.

[linphone] / mediastreamer2

diff --git a/mediastreamer2 b/mediastreamer2
index 8596ed9017c0d9b9c63c758d7628bac1fc07efd7..f6c51a11f1ef1156c6f768b8466698a2305a2188 160000 (submodule)
--- a/mediastreamer2
+++ b/mediastreamer2
@@ -1 +1 @@
-Subproject commit 8596ed9017c0d9b9c63c758d7628bac1fc07efd7
+Subproject commit f6c51a11f1ef1156c6f768b8466698a2305a2188