ath79: fix wmac initialization for Zyxel NBG6716

ath79: fix wmac initialization for Zyxel NBG6716

2.4 GHz Wifi on ath79 is set up in 10-ath9k-eeprom, but in ar71xx
it was done with ath79_register_wmac.

Thus, the following errors are observed on the device:
ath: phy1: Unable to initialize hardware; initialization status: -5
ath9k 18100000.wmac: failed to initialize device
ath9k: probe of 18100000.wmac failed with error -5

This patch changes the ath79 support to properly use wmac as well.
This will also require fixing the MAC address in a different way.

Signed-off-by: Guillaume Lefebvre <guillaume@zelig.ch>
[several adjustments to 10-fix-wifi-mac, use correct MAC address,
rewrite commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>