MCS: move Coding Scheme enum outside of class definition

Move generic MCS enum to C header file to simplify further modifications
to GprsCodingScheme class in follow-up patches. This also allows us
to use standard libosmocore value_sting functions in upcoming patches
for IA Rest Octet encoding/decoding.

Related: OS#3014
Change-Id: I993b49d9a82b8c7ad677d52d11003794aeabe117
diff --git a/src/gprs_ms.h b/src/gprs_ms.h
index 857f0c9..0a9efb8 100644
--- a/src/gprs_ms.h
+++ b/src/gprs_ms.h
@@ -86,7 +86,7 @@
 	uint8_t egprs_ms_class() const;
 	void set_ms_class(uint8_t ms_class);
 	void set_egprs_ms_class(uint8_t ms_class);
-	void set_current_cs_dl(GprsCodingScheme::Scheme scheme);
+	void set_current_cs_dl(CodingScheme scheme);
 
 	GprsCodingScheme current_cs_ul() const;
 	GprsCodingScheme current_cs_dl() const;