ms: validate codel enabled against proper define

This is a cosmetic change, since LLC_CODEL_DISABLE is defined as 0.
It clarifies though the meaning of the if condition.

Change-Id: I7c4bda5977a94c2aaf01ca368570f80fd6ebae00
diff --git a/src/gprs_ms.c b/src/gprs_ms.c
index c10eda7..3167dee 100644
--- a/src/gprs_ms.c
+++ b/src/gprs_ms.c
@@ -119,7 +119,7 @@
 	if (ms->bts)
 		codel_interval = the_pcu->vty.llc_codel_interval_msec;
 
-	if (codel_interval) {
+	if (codel_interval != LLC_CODEL_DISABLE) {
 		if (codel_interval == LLC_CODEL_USE_DEFAULT)
 			codel_interval = GPRS_CODEL_SLOW_INTERVAL_MS;
 		ms->codel_state = talloc(ms, struct gprs_codel);