thunderbolt: Fix memory leak in margining

Memory for the usb4->margining needs to be relased for the upstream port
of the router as well, even though the debugfs directory gets released
with the router device removal. Fix this.

Fixes: d0f1e0c2

 ("thunderbolt: Add support for receiver lane margining")
Cc: stable@vger.kernel.org
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>