ath9k: add new function ath9k_hw_read_array

REG_READ generate most overhead on usb bus. It send and read micro packages
and reduce usb bandwidth. To reduce this overhead we should read in batches.

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