host: Wake up stalled conns when a conn is dropped

When a connection is dropped, it may free up some ACL packet capacity in
the controller.  When this happens, we need to wake up all stalled
connections.

We want to wake up connections every time the ACL data buffer capcity
increases (whenever `ble_hs_hci_add_avail_pkts()` is called).

nimble/host/src/ble_hs_hci_evt.c

@@ -166,6 +166,12 @@ ble_hs_hci_evt_disconn_complete(uint8_t event_code, uint8_t *data, int len)
 
     ble_gap_rx_disconn_complete(&evt);
 
+    /* The connection termination may have freed up some capacity in the
+     * controller for additional ACL data packets.  Wake up any stalled
+     * connections.
+     */
+    ble_hs_wakeup_tx();
+
     return 0;
 }