thunderbolt: Clear hops before overwriting

Zero hops in tb_path_activate before writing a new path.

This fixes the following scenario:
 - Boot with a coldplugged device
 - Unplug device
 - Plug device back in
 - PCI hotplug fails

The hotplug operation fails because our new path matches the (now
defunct) path which was setup by the firmware for the coldplugged
device. By writing zeros before writing our path configuration we can
force thunderbolt to retrain the path.

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