Bluetooth: hci_conn: use mod_delayed_work for active mode timeout

hci_conn_enter_active_mode() uses queue_delayed_work() with the
intention that the work will run after the given timeout. However,
queue_delayed_work() does nothing if the work is already queued, so
depending on the link policy we may end up putting the connection
into idle mode every hdev->idle_timeout ms.

Use mod_delayed_work() instead so the work is queued if not already
queued, and the timeout is updated otherwise.

Signed-off-by: Stefan Sørensen <ssorensen@roku.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>