HS 2.0: Deauthenticate STA on deauth-imminent more quickly if no URL

HS 2.0: Deauthenticate STA on deauth-imminent more quickly if no URL

When the RADIUS server requests a STA to be deauthenticated imminently
without providing a reason URL, there is no need to allow the STA spend
any additional time associated. Deauthenticate the STA immediately after
it has ACK'ed the WNM-Notification frame indicating imminent
deauthentication or at latest two seconds after having processes the
Access-Accept message.

Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>