staging: rtl8723au: Use nl80211_iftype instead of driver's own custom types

In addition move rtw_set_802_11_infrastructure_mode23a() to
ioctl_cfg80211.c and declare it static.

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