ath9k_hw: extend ath9k_hw_set_txpowerlimit to test channel txpower

ath9k_hw_set_txpowerlimit gets an extra boolean parameter that - if set -
causes the rate txpower table and the regulatory limit to be calculated
and stored, without changing hardware registers.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>