ath9k: Fix ASPM for AR9462

If the L1 entrance latency is not calibrated properly
in the EEPROM in WB222 boards, there could be problems
in connectivity. Check and correct the calibrated value
if it doesn't match the optimal value for WB222, 4us.

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