staging: rtl8723au: ioctl_cfg80211.c: Convert rtw_get_wps_ie23a() to cfg80211_find_vendor_ie()

Given that the IE len is stored in a u8, there is no point having a
512 buffer to store it, nor check if the size exceeds this size.

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