thunderbolt: Make the DP tunnel activation callback mandatory

In the Linux kernel, the following vulnerability has been resolved:

thunderbolt: Make the DP tunnel activation callback mandatory

tb_tunnel_alloc_dp() takes an optional callback which is run from
dprx_work once the DPRX capabilities read has completed. Without that
callback tb_dp_dprx_start() reads the capabilities synchronously and
never queues the work. It however always takes a tunnel reference which
is only dropped by dprx_work itself or by tb_dp_dprx_stop() when
cancel_delayed_work() actually canceled that work. That reference is
thus leaked for every tunnel without a callback.

The only tunnels without one are those from tb_tunnel_discover_dp(),
which are activated again when restoring from hibernation.
Pass the callback to tb_tunnel_discover_dp() as well and drop the
synchronous path such that the DPRX capabilities are always read from
dprx_work. Hibernation restore then also no longer blocks for up to 12
seconds while waiting for that read to complete.

Also fix up the KUnit tests.