thunderbolt: Add trivial .shutdown

On my machine, a kexec with this driver loaded in the old kernel causes
a very long delay on boot in the kexec'ed kernel, most likely due to
unclean shutdown prior to that.

Unloading thunderbolt driver prior to kexec allows kexec to work as fast
as regular kernel boot, as well as adding this .shutdown pointer.

Shutting a device prior to the shutdown completely is always a good idea
IMHO to help with kexec, and this one-liner patch implements it.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>