bsc_api/NITB: If TCH/H channel is used, indicate it to bsc_api

If we don't do this, OsmoNITB will send an assignment command from
a TCH/H to another TCH/H without any need.
diff --git a/openbsc/src/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index 43e83c7..74dc27f 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -2878,7 +2878,8 @@
 {
 	struct gsm_mncc *mode = arg;
 
-	return gsm0808_assign_req(trans->conn, mode->lchan_mode, 1);
+	return gsm0808_assign_req(trans->conn, mode->lchan_mode,
+		trans->conn->lchan->type != GSM_LCHAN_TCH_H);
 }
 
 static struct downstate {