thunderbolt: Do not overwrite error code when domain adding fails

If the Thunderbolt domain adding fails for some reason we currently
always return -EIO instead of the real error code. To make debugging
easier return the actual error code instead.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>