thunderbolt: Setup control channel

Add struct tb which will contain our view of the thunderbolt bus.  For
now it just contains a pointer to the control channel and a workqueue
for hotplug events.

Add thunderbolt_alloc_and_start() and thunderbolt_shutdown_and_free()
which are responsible for setup and teardown of struct tb.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>