ath9k: fix bug in tx power

The ath9k driver subtracts 3 dBm to the txpower as with two radios the
signal power is doubled.
The resulting value is assigned in an u16 which overflows and makes
the card work at full power.

Cc: stable@kernel.org
Signed-off-by: Matteo Croce <matteo@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>