mgcp: Fix SDP formatting of fmtp_extra (Coverity)

Currently when ftmp_extra is set, a doubled a=rtpmap line is emitted
instead of the fmtp_extra info.

This patch fixes replaces the formerly copied and pasted but not
modified snprintf parameters by the correct ones.

Fixes: Coverity CID 1220873
Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index f0457d1..6e48949 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -287,8 +287,7 @@
 
 		if (fmtp_extra) {
 			nchars = snprintf(sdp_record + len, size - len,
-					  "a=rtpmap:%d %s\r\n",
-					  payload_type, audio_name);
+					  "%s\r\n", fmtp_extra);
 
 			if (nchars < 0 || nchars >= size - len)
 				goto buffer_too_small;