WPS: Fix regression in post-WPS scan optimization

WPS: Fix regression in post-WPS scan optimization

Commit 3c85f144ce7aa3967f881a7584a5b125061ca436 fixed issues with P2P
Action frame TX after disconnection by clearing wpa_s->assoc_freq.
This resulted in a regression to the post-WPS scan optimization that
used wpa_s->assoc_freq to enable fast single-channel scan. Fix this by
copying wpa_s->assoc_freq to a local variable before calling
wpa_supplicant_deauthenticate() that ends up calling
wpa_supplicant_mark_disassoc() which will clear assoc_freq.

Reported-by: Angie Chinchilla <angie.v.chinchilla@intel.com>
intended-for: hostap-1
Signed-hostap: Jouni Malinen <j@w1.fi>