ath9k_hw: Initvals update for AR9462

MSI is enabled by default for most of the 4th generation
chips. Add this for AR9462 - this fixes PowerSave operation,
the chip was not entering Network-Sleep mode earlier.
With proper powering down of the MAC now, power consumption
in associated state is reduced considerably.

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