ath9k: ath9k_hw_init_macaddr should not overwrite valid MAC addresses

Currently setting the MAC address via ath9k_platform_data works only due
to the order in which init.c sets common->macaddr, which is done after
ath9k_hw_init_macaddr was executed. It would be better if the latter
was independent of the order in which it's being called.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>