[gsm48] Allow to send the assignment command on a different lchan

Change the signature to take the lchan were the message is supposed
to be sent and the lchan which is supposed to be assigned.
diff --git a/openbsc/src/gsm_04_08_utils.c b/openbsc/src/gsm_04_08_utils.c
index d5785f9..e95d426 100644
--- a/openbsc/src/gsm_04_08_utils.c
+++ b/openbsc/src/gsm_04_08_utils.c
@@ -567,7 +567,7 @@
 }
 
 /* Chapter 9.1.2: Assignment Command */
-int gsm48_send_rr_ass_cmd(struct gsm_lchan *lchan, u_int8_t power_command)
+int gsm48_send_rr_ass_cmd(struct gsm_lchan *dest_lchan, struct gsm_lchan *lchan, u_int8_t power_command)
 {
 	struct msgb *msg = gsm48_msgb_alloc();
 	struct gsm48_hdr *gh = (struct gsm48_hdr *) msgb_put(msg, sizeof(*gh));
@@ -576,7 +576,7 @@
 
 	DEBUGP(DRR, "-> ASSIGNMENT COMMAND tch_mode=0x%02x\n", lchan->tch_mode);
 
-	msg->lchan = lchan;
+	msg->lchan = dest_lchan;
 	gh->proto_discr = GSM48_PDISC_RR;
 	gh->msg_type = GSM48_MT_RR_ASS_CMD;