channel_mode_from_lchan(): Add missing break statement

GSM48_CMODE_DATA_6k0 was not properly terminated and thus resulted in a
bug.

Change-Id: I4000f06d0b49c4afb0446beddd150521c4ba3cf0
Fixes: Coverity CID 148207
diff --git a/openbsc/src/libbsc/abis_rsl.c b/openbsc/src/libbsc/abis_rsl.c
index 2c016a6..8a6b111 100644
--- a/openbsc/src/libbsc/abis_rsl.c
+++ b/openbsc/src/libbsc/abis_rsl.c
@@ -453,6 +453,7 @@
 			     lchan->csd_mode);
 			return -EINVAL;
 		}
+		break;
 	default:
 		LOGP(DRSL, LOGL_ERROR,
 		     "unsupported lchan->tch_mode %u\n",