ath9k: fix two more bugs in tx power

This is the same fix as

   commit 84105160
   Author: Matteo Croce <technoboy85@gmail.com>
   Date:   Fri Dec 3 02:25:08 2010 +0100

   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.

in two more places. I grepped the ath tree and didn't find any others.

Cc: stable@kernel.org
Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>