mgcp: Align the variable naming with Appendix A of RFC 3550

Align the naming inside the mgcp_rtp_state with the naming inside
the 'source' struct of the appendix. Make first_seq_no/base_seq
a uint16_t. This is removing rules for alignments and reduces the
struct from 40 bytes to 36.
diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index d7e51a0..e0086fa 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -46,9 +46,11 @@
 
 	uint32_t orig_ssrc;
 	uint32_t ssrc;
-	int first_seq_no;
-	uint16_t seq_no;
+
+	uint16_t base_seq;
+	uint16_t max_seq;
 	int seq_offset;
+
 	uint32_t last_timestamp;
 	int32_t  timestamp_offset;
 };