ath10k: disable sta keepalive

Firmware revisions providing sta keepalive service
have it enabled by default.

mac80211 already does idle connection polling so
it makes no sense to duplicate this in ath10k.
mac80211 wouldn't even know of the offloaded
keepalive NullFunc frames.

This prevents sending out some extraneous frames
on the air.

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>