[lchan] RSL and RR need the multirate config, place it in the lchan

Both GSM 04.08 RR and GSM 08.58 RSL need the multirate config
in the channel modify. Place the config in the lchan, change
the gsm48 methods to not take the argument, change the RSL
implementation to make use of it with the right IE.

The other code should use the t(l)v_put routines as well but
were left untouched for now.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 4d352bc..1f82354 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -3139,7 +3139,7 @@
 {
 	struct gsm_mncc *mode = arg;
 
-	return gsm48_lchan_modify(trans->lchan, mode->lchan_mode, NULL);
+	return gsm48_lchan_modify(trans->lchan, mode->lchan_mode);
 }
 
 static struct downstate {