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
diff --git a/src/mgcp/mgcp_network.c b/src/mgcp/mgcp_network.c
index 3bbc600..36ef932 100644
--- a/src/mgcp/mgcp_network.c
+++ b/src/mgcp/mgcp_network.c
@@ -197,7 +197,7 @@
 	if (udelta < RTP_MAX_DROPOUT) {
 		if (seq < state->max_seq)
 			state->cycles += RTP_SEQ_MOD;
-	} else if (udelta <= RTP_SEQ_MOD + RTP_MAX_MISORDER) {
+	} else if (udelta <= RTP_SEQ_MOD - RTP_MAX_MISORDER) {
 		LOGP(DMGCP, LOGL_NOTICE,
 			"RTP seqno made a very large jump on 0x%x delta: %u\n",
 			ENDPOINT_NUMBER(endp), udelta);