rsl: Rename the reason to release_mode and use the enum value

The API with 0 and 1 as legal values is hard to understand. Use the
recently introduced enum values instead.
diff --git a/openbsc/src/libbsc/chan_alloc.c b/openbsc/src/libbsc/chan_alloc.c
index 69b2625..fa369a2 100644
--- a/openbsc/src/libbsc/chan_alloc.c
+++ b/openbsc/src/libbsc/chan_alloc.c
@@ -345,7 +345,7 @@
 	}
 
 	lchan->sacch_deact = 0;
-	lchan->release_mode = 0;
+	lchan->release_mode = RSL_REL_NORMAL;
 
 	/* FIXME: ts_free() the timeslot, if we're the last logical
 	 * channel using it */
@@ -424,7 +424,7 @@
 }
 
 /* Consider releasing the channel now */
-int lchan_release(struct gsm_lchan *lchan, int sacch_deact, int mode)
+int lchan_release(struct gsm_lchan *lchan, int sacch_deact, enum rsl_rel_mode mode)
 {
 	DEBUGP(DRLL, "%s starting release sequence\n", gsm_lchan_name(lchan));
 	rsl_lchan_set_state(lchan, LCHAN_S_REL_REQ);