wil6210: implement cfg80211 probe_client() op

Access point require this API to check peer alive status.
Assume peer is alive when it is connected, because
firmware implements keep alive checks and will disconnect
peer if it is not alive.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>