staging: rtl8723au: OnAssocReq23a(): Clean up and use more ieee80211.h

This converts it to use struct ieee80211_mgmt and cfg80211_find_ie().
In addition fix a potential buffer overrun in the last loop searching
through list WLAN_EID_VENDOR_SPECIFIC elements.
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>