thunderbolt: No need to warn in TB_CFG_ERROR_INVALID_CONFIG_SPACE

This may be returned for example when accessing some of the vendor
specific capabilities. It is not fatal by any means so instead of WARN()
just log it as debug level. The caller gets error back anyway and is
expected to handle it accordingly.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>