staging: refresh TODO for rtl8712

People should not waste time and energy working on this staging driver.
At least four drivers were written for this hardware:
 https://marc.info/?l=linux-wireless&m=138358275410975
And there is a replacement using the kernel wireless stack at:
 https://github.com/chunkeey/rtl8192su


Also a fullmac/cfg80211 driver(r92su) is available.

Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Florian Schilhabel <florian.c.schilhabel@googlemail.com>
Cc: Christian Lamparter <chunkeey@googlemail.com>
Cc: Joshua Roys <Joshua.Roys@gtri.gatech.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linux Driver Project Developer List <driverdev-devel@linuxdriverproject.org>
Cc: Linux Wireless <linux-wireless@vger.kernel.org>
Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>