Use proper undefined hci error status code

Bug: 163134718
Tag: #refactor
Test: act.py -tc BleCocTest
Test: ble paired 2 phones
Test: classic paired Bose SoundLink

Change-Id: I2a95b74e8a8a3b31ec63e2ef64070ab4c2d374f1
diff --git a/system/stack/acl/btm_acl.cc b/system/stack/acl/btm_acl.cc
index e061f9c..b730bab 100644
--- a/system/stack/acl/btm_acl.cc
+++ b/system/stack/acl/btm_acl.cc
@@ -136,7 +136,7 @@
 void btm_acl_init(void) {
   /* Initialize nonzero defaults */
   btm_cb.acl_cb_.btm_def_link_super_tout = HCI_DEFAULT_INACT_TOUT;
-  btm_cb.acl_cb_.acl_disc_reason = 0xff;
+  btm_cb.acl_cb_.acl_disc_reason = HCI_ERR_UNDEFINED;
 
   btm_cb.acl_cb_.btm_acl_pkt_types_supported =
       HCI_PKT_TYPES_MASK_DH1 + HCI_PKT_TYPES_MASK_DM1 + HCI_PKT_TYPES_MASK_DH3 +