mgcp: Calculate the packet loss as of Appendix A of RFC 3550

Calculate the expected packages and packet loss as of RFC 3550.
The values should be clamped but our packet loss counter is 32
bits and not 24 and we should clamp at other values but I am
waiting for some issues first before dealing with that.
diff --git a/openbsc/tests/mgcp/mgcp_test.ok b/openbsc/tests/mgcp/mgcp_test.ok
index 5e0e47c..e61c0bc 100644
--- a/openbsc/tests/mgcp/mgcp_test.ok
+++ b/openbsc/tests/mgcp/mgcp_test.ok
@@ -8,4 +8,5 @@
 Testing SHORT3
 Testing SHORT4
 Testing DLCX
+Testing packet loss calculation.
 Done