thunderbolt: ACPI: Use the helper fwnode_find_reference()

Replacing the direct fwnode_property_get_reference_args()
call will this wrapper function.

No functional changes intended.

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