libertas: add an cfg80211 interface for powersaving

This patch adds an interface for handling commands like
iwconfig wlanX power on/off. Such an interface formerly existed
when the driver used wext.

While performance with sdio in polling mode without using
powersave mode is quite bad, powersaving mode is unusable,
so do not enable it under such conditions.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>