FreeTDM: Fix implicit conversion between enumeration types error

Full clang error message:

  error: implicit conversion from enumeration type 'ftdm_bool_t' to
  different enumeration type 'ftdm_status_t' [-Werror,-Wconversion]
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

libs/freetdm/src/ftmod/ftmod_misdn/ftmod_misdn.c

@@ -392,7 +392,7 @@ static ftdm_status_t misdn_event_queue_reset(struct misdn_event_queue *queue)
 	return FTDM_SUCCESS;
 }
 
-static ftdm_status_t misdn_event_queue_has_data(const struct misdn_event_queue *queue)
+static ftdm_bool_t misdn_event_queue_has_data(const struct misdn_event_queue *queue)
 {
 	if (!queue)
 		return FTDM_FALSE;