ath9k: ath9k_hw_set_txpowerlimit sets previous txpower

In commit 9c204b46
(ath9k_hw: do not limit initial tx power to 20 dbm),
setting of txpower was broken.

This patch fixes it by initializing reg_pwr from the new
power limit, not the previous value.

Signed-off-by: Bill Jordan <bjordan@rajant.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>