tbf: Move the current CS field to GprsMs

Currently the current CS value is stored in the cs field of
gprs_rlcmac_tbf and initialised when it is used the first time.

This commit adds separate fields for UL and DL CS values to the
GprsMs class and provides corresponding getter methods for GprsMs and
gprs_rlcmac_tbf.

Ticket: #1739
Sponsored-by: On-Waves ehf
diff --git a/src/tbf.h b/src/tbf.h
index c5bb900..f50c489 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -153,6 +153,7 @@
 	void set_ta(uint8_t);
 	uint8_t ms_class() const;
 	void set_ms_class(uint8_t);
+	uint8_t current_cs() const;
 	gprs_llc_queue *llc_queue();
 	const gprs_llc_queue *llc_queue() const;
 
@@ -198,8 +199,6 @@
 		int rssi_num; /* number of rssi values added since rssi_tv */
 	} meas;
 
-	uint8_t cs; /* current coding scheme */
-
 	/* these should become protected but only after gprs_rlcmac_data.c
 	 * stops to iterate over all tbf in its current form */
 	enum gprs_rlcmac_tbf_state state;