thunderbolt: Keep link as asymmetric if preferred by hardware

In case of the link is brought up as asymmetric (due to hardware preference), we
honor that and don't transition it to symmetric, unless a router with symmetric
link got plugged below, in the topology (and a bandwidth allows transition to
symmetric).

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