staging: rtl8723au: rtw_get_cipher_info(): Convert to use cfg80211_* API

Use cfg80211_find_ie() and cfg80211_find_vendor_ie() rather than own
hacks.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>