udev: Avoid warning messages

Avoids these messages being printed in the journal:

    systemd-udevd[563]: /etc/udev/rules.d/70-protect-boot-medium-for-udisks.rules:9 ENV key takes '==', '!=', '=', or '+=' operator, assuming '=', but please fix it.
    systemd-udevd[563]: /etc/udev/rules.d/70-protect-boot-medium-for-udisks.rules:16 ENV key takes '==', '!=', '=', or '+=' operator, assuming '=', but please fix it.