ath6kl: fix firmware start address for ar6003 hw2.0

Sangwook found out that commit 639d0b89 ("ath6kl: read firmware start
address from hardware") broke firmware boot on ar6003 hw2.0 as it seems
it's not posible to automatically query the address from hardware. So
we need to hardcode the address for hw2.0.

Reported-by: Sangwook Lee <sangwook.lee@linaro.org>
Tested-by: Sangwook Lee <sangwook.lee@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>