thunderbolt: Do not report errors if on-board retimers are found

Currently we return an error even if on-board retimers are found and
that's not expected. Fix this to return an error only if there was one
and 0 otherwise.

Fixes: 1e56c88a ("thunderbolt: Runtime resume USB4 port when retimers are scanned")
Cc: stable@vger.kernel.org
Signed-off-by: Utkarsh Patel <utkarsh.h.patel@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>