ath9k: merge power correction constants

The existing constants are used for reduction/increase
tx power level on devices with 2x2 and 3x3 chainmask.

Both reduction and increase must use the same value, so
it makes no sense to use separate constants for them.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>