Logic was inverted when changing from string check to file check.
Fix it.
Fixes: 8592602d0a ("base-files: Really check path in get_mac_binary")
Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 6ed3349308)