mgcp: Fix the vary large jump detection in the code

GCC 3.x on PowerPC correctly highlights that the code is fishy.
Re-reading the RFC 3550 shows that we should subtract it and then
we are in the 16bit range. The probation and re-sync code is still
missing.

GCC:
mgcp/mgcp_network.c:200: warning: comparison is always true due to limited range of data type
1 file changed