thunderbolt: Enable/disable sideband depending on USB4 port offline mode

When USB4 port is in offline mode (this mean there is no device
attached) we want to keep the sideband up to make it possible to
communicate with the retimers. In the same way there is no need to
enable sideband transactions when the USB4 port is not offline as they
are already up.

For this reason make the enabling/disabling depend on the USB4 port
offline status.

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