Replace magic number with define

Value 4 used as magic number by both OpenBSC and OsmoBTS so it make
sense to add it to shared header. See
ebb483b69a5319e522ba5f713e9cb6f68a814a6a in osmo-bts for details.

Change-Id: I9c6ad68f4c6aa72d39ec7e5a6968b36ec20e79f4
diff --git a/openbsc/src/libcommon/gsm_data_shared.c b/openbsc/src/libcommon/gsm_data_shared.c
index b8853ad..df204fe 100644
--- a/openbsc/src/libcommon/gsm_data_shared.c
+++ b/openbsc/src/libcommon/gsm_data_shared.c
@@ -579,7 +579,7 @@
 		 * here. This should never be sent in an RSL message.
 		 * See osmo-bts-xxx/oml.c:opstart_compl().
 		 */
-		if (lchan_nr == 4)
+		if (lchan_nr == CCCH_LCHAN)
 			chan_nr = 0;
 		else
 			OSMO_ASSERT(lchan_nr < 4);