MCS: add mcs_is_*() helpers

In preparation for Channel Coding Command encoder in follow-up patches
let's add necessary helpers. Those are similar to previously used
helpers from GprsCodingScheme class but without CamelCase and with less
typo chances between Gprs and Egprs cases.

Change-Id: I6699cbc8d7ae766fa4d2b3d37e5f9ff1cf158b7e
diff --git a/src/encoding.cpp b/src/encoding.cpp
index e21a7fc..9920b46 100644
--- a/src/encoding.cpp
+++ b/src/encoding.cpp
@@ -1605,12 +1605,12 @@
 	gprs_llc *llc, int *offset, int *num_chunks,
 	uint8_t *data_block, bool is_final, int *count_payload)
 {
-	if (cs.isGprs())
+	if (mcs_is_gprs(cs))
 		return rlc_data_to_dl_append_gprs(rdbi,
 			llc, offset, num_chunks, data_block, is_final,
 			count_payload);
 
-	if (cs.isEgprs())
+	if (mcs_is_edge(cs))
 		return rlc_data_to_dl_append_egprs(rdbi,
 			llc, offset, num_chunks, data_block, is_final,
 			count_payload);