ath6kl: implement ath6kl_cfg80211_suspend()

This is in preparation for cutpower suspend feature. HIF layer makes
the decision based on information provided by cfg80211 and what hardware
actually supports. Then it calls ath6kl_cfg80211_suspend() to enable
the chosen mode.

Functionality should be the same, this is just preparation for
more suspend modes (cutpower and wow).

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>