ath9k: Fix TX power calculation

The commit, "ath9k_hw: Fix incorrect Tx control power in AR9003 template"
fixed the incorrect values in the eeprom templates, but if
boards have already been calibrated with incorrect values,
they would still be using the wrong TX power. Fix this by assigning
a default value in such cases.

Cc: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>