ath79: rename Mikrotik RB 922UAGS-5HPacD mtd partition

ath79: rename Mikrotik RB 922UAGS-5HPacD mtd partition

In RouterBOARD parlance there never was an "art" partition.
This partition has always been named 'hard_config' on ar71xx.

This partition contains more than just ART (Atheros Radio Test) data. It
includes the hardware description (product code, serial, board
identifier, name, hardware options, MAC address), as well as other bits
affecting the operation of RouterBoot.
To avoid confusion with regular ART data, this partition is renamed in
line with historical ar71xx and ramips nomenclature as 'hard_config'.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
[minor commit title/message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>