ofw_firmware: Only match if there is no compatible

If there is a compatible string it likely means that the firmware needs
a dedicated driver (like on RPI*).

PR: 273087
Tested-by: Mark Millard <marklmi26-fbsd@yahoo.com>
Sponsored by: Beckhoff Automation GmbH & Co. KG
Fixes: fdfd3a90b6ce ("ofw: Add a ofw_firmware driver")