ath10k: enable ASPM

It is actually safe to enable ASPM after the
device is booted up.

This reduces power drain of QCA61X4 when driver is
simply loaded (no interface is up) from 31mA to
14mA. QCA988X wasn't measured but doesn't seem to
regress in any other way.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>