ath9k: fetch calibration data via nvmem subsystem

On most embedded ath9k devices (like range extenders,
routers, accesspoints, ...) the calibration data is
stored in a MTD partitions named "ART", or "caldata"/
"calibration".

Since commit 4b361cfa

 ("mtd: core: add OTP nvmem provider support"):
All MTD partitions are all automatically available through
the nvmem subsystem.

This feature - together with an nvmem cell definition either
in the platform data or via device-tree allows drivers to get
the data necessary for initializing the WIFI, without having
to wait around for the filesystem and userspace to do
the extractions.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/f9b732b50a3453fadf3923cc75d365bae3505fe7.1630157099.git.chunkeey@gmail.com