When SAP is restarted, all connected station are disassociated
and deauthenticated. But Cfg80211 layer return immediately.
This result in some station not getting really disassociated.
This update fixes this situation by waiting for deauth completion
before proceeding with next station deauth.

Change-Id: I09551c813736c1b75a1b70ade73011ad0494875e
CRs-fixed: 767351