thunderbolt: Set tb->root_switch to NULL when domain is stopped

thunderbolt: Set tb->root_switch to NULL when domain is stopped

Similarly what we do with the firmware connection manager. This makes
tb_xdp_handle_request() return error to the remote host. However, we
need to make sure we keep the uuid alive so that we can reply until the
whole domain is released.

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