ath10k: enable sta idle power save

Enable sta power save in fw for the targets that
supports idle power save. The idle ps enable command
will be ignored by the firmware which does not support
this feature.

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>