thunderbolt: Set lane bonding bit only for downstream port

Fix the lane bonding procedure to follow the steps described in USB4
Connection Manager guide. Hence, set the lane bonding bit only for
downstream port. This is needed for certain ASMedia device, otherwise
lane bonding fails and the device disconnects.

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