wil6210: fix disconnect 1 STA in AP

When disconnecting single STA in AP, it might be that STA
in question is already disconnected. In this case, need to do nothing.
Previously, it was mis-interpreted as "disconnect all"

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