core: Fix sequencing number issues

Sequencing in MCTP allows SOM to declare a completely new sequence
number. Also add an implementation that handles messages that are
neither SOM or EOM.

Minor changes from Jeremy Kerr <jk@ozlabs.org>

Signed-off-by: Ed Tanous <ed.tanous@intel.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/libmctp.h b/libmctp.h
index a47eb35..5c0c9f4 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -25,7 +25,7 @@
 #define MCTP_HDR_FLAG_SOM	(1<<7)
 #define MCTP_HDR_FLAG_EOM	(1<<6)
 #define MCTP_HDR_FLAG_TO	(1<<3)
-#define MCTP_HDR_SEQ_SHIFT	(5)
+#define MCTP_HDR_SEQ_SHIFT	(4)
 #define MCTP_HDR_SEQ_MASK	(0x3)
 #define MCTP_HDR_TAG_SHIFT	(0)
 #define MCTP_HDR_TAG_MASK	(0x7)