thunderbolt: Handle lane bonding of Gen 4 XDomain links properly

Gen 4 links come up as bonded already so we are not supposed to initiate
lane bonding on them. However, we should still update the port
structures accordingly. Split these into their own functions to make it
easier to follow.

Signed-off-by: Gil Fine <gil.fine@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>