staging: refresh TODO for rtl8723au

People should not waste time and energy working on this staging driver.
A replacement(rtl8xxxu) using the kernel wireless stack already was merged
in the 4.3 kernel.

Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>