Convert the calibration data reference for the ath9k radio to an
nvmem-cell, replacing the downstream mtd-cal-data property.
Since the 'art' label is no longer used, it can be dropped.
Cc: Sebastian Schaper <openwrt@sebastianschaper.net>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
849ffbd5ab