ath9k: remove return value from ath9k_hw_init_macaddr

ath9k_hw_init_macaddr unconditionally returns 0 in all cases, making the
return value unnecessary.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>