Fix mcs_is_valid(): UNKNOWN value is not a valid (M)CS

Also add a few more asserts to make sure a valid CS/MCS is passed in
some placed where we expect (M)CS to be set.

Change-Id: I0a973e10cd9477f72d8bd47a06048414b33ae96a
diff --git a/src/coding_scheme.c b/src/coding_scheme.c
index e39ed2b..27241d1 100644
--- a/src/coding_scheme.c
+++ b/src/coding_scheme.c
@@ -163,7 +163,7 @@
 
 bool mcs_is_valid(enum CodingScheme cs)
 {
-	return UNKNOWN <= cs && cs <= MCS9;
+	return UNKNOWN < cs && cs <= MCS9;
 }
 
 bool mcs_is_compat_kind(enum CodingScheme cs, enum mcs_kind mode)