mgcp: Move the "codec" params to a struct

We might be offered multiple codecs by the remote and need to
switch between them once we receive data. Do this by moving it
to a struct so we can separate between proposed and current
codec. In SDP we can have multiple codecs but a global ptime.
The current code doesn't separate that clearly instead we write
it to the main codec.
diff --git a/openbsc/src/libmgcp/mgcp_vty.c b/openbsc/src/libmgcp/mgcp_vty.c
index 9ae0bc9..b29eb6b 100644
--- a/openbsc/src/libmgcp/mgcp_vty.c
+++ b/openbsc/src/libmgcp/mgcp_vty.c
@@ -147,6 +147,8 @@
 static void dump_rtp_end(const char *end_name, struct vty *vty,
 			struct mgcp_rtp_state *state, struct mgcp_rtp_end *end)
 {
+	struct mgcp_rtp_codec *codec = &end->codec;
+
 	vty_out(vty,
 		"  %s%s"
 		"   Timestamp Errs: %d->%d%s"
@@ -160,10 +162,10 @@
 		state->in_stream.err_ts_counter,
 		state->out_stream.err_ts_counter, VTY_NEWLINE,
 		end->dropped_packets, VTY_NEWLINE,
-		end->payload_type, end->rate, end->channels, VTY_NEWLINE,
-		end->frame_duration_num, end->frame_duration_den, VTY_NEWLINE,
+		codec->payload_type, codec->rate, codec->channels, VTY_NEWLINE,
+		codec->frame_duration_num, codec->frame_duration_den, VTY_NEWLINE,
 		end->frames_per_packet, end->packet_duration_ms, VTY_NEWLINE,
-		end->fmtp_extra, end->audio_name, end->subtype_name, VTY_NEWLINE,
+		end->fmtp_extra, codec->audio_name, codec->subtype_name, VTY_NEWLINE,
 		end->output_enabled, end->force_output_ptime, VTY_NEWLINE);
 }