ath9k_hw: Fix tx power settings for AR9003

Retriving tx power for 2x2 and 3x3 chainmask is not handled
properly. While calculating tx power for 2x2, 3 dBm was reduced
and for 3x3, 5 dBm was reduced which should be added back when
retriving.

Cc: Paul Stewart <pstew@google.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>