ath9k: move all ath9k_platform_data initialization into one function

No functional changes, this simply makes the code easier to understand
because all initialization based on ath9k_platform_data is now within
one function.

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