base-files: Fix path check in get_mac_binary

Logic was inverted when changing from string check to file check.
Fix it.

Fixes: 8592602d0a88 ("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 6ed3349308b24a6bac753643970a1f9f56ff6070)

package/base-files/files/lib/functions/system.sh

@@ -4,7 +4,7 @@ get_mac_binary() {
 	local path="$1"
 	local offset="$2"
 
-	if [ -e "$path" ]; then
+	if ! [ -e "$path" ]; then
 		echo "get_mac_binary: file $path not found!" >&2
 		return
 	fi