ath79: remove bs-partition ro-flag for UniFi AC devices

This removes the read-only flag from the bs (bootselect) partition
on UniFi AC devices. This allows to correct the indicator from which
partition the device is booting its kernel from.

See also:
 - freifunk-gluon/gluon#1301
 - https://bugs.lede-project.org/index.php?do=details&task_id=662

Signed-off-by: Christoph Krapp <achterin@googlemail.com>