ath9k: Enable WoW only for AR9462

The only card with which WoW has been tested and verified is
AR9462. Do not enable it for all cards since WoW is really quirky
and needs to be tested properly with each chip.

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