Change reason code for AP mode BSS termination to 3

Change reason code for AP mode BSS termination to 3

Use reason code 3 (Deauthenticated because sending STA is leaving)
instead of reason code 2 (Previous authentication ot valid) in the
Deauthentication frame sent at the time a BSS is terminated. This is
more accurate reason for the message and allows the stations to get
better understand on why they were disconnected. This can be useful
especially for P2P clients that can use this as a signal that the
P2P group session was ended per P2P specification section 3.2.9.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>