ath9k_common: always update value in ath9k_cmn_update_txpow

In some cases the limit may be the same as reg->power_limit, but the
actual value that the hardware uses is not up to date. In that case, a
wrong value for current tx power is tracked internally.
Fix this by unconditionally updating it.

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