thunderbolt: Log the link as TBT instead of TBT3

The upstream port can be connected to any previous generation
Thunderbolt port so logging as "TBT" is more accurate than "TBT3.

No functional changes.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>