ath9k: ar9271_hw_pa_cal: use REG_READ_ARRAY

insted of reading each register separatly
and waste 4ms on each operation, we can
use one shot read.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>