staging: rtl8723au: Move rtw_get_cur_max_rate23a() to ioctl_cfg80211.c

This function has only one caller, so move it home and declare it static

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