CTS protection was not working properly because the HW still need
RTS flag to be asserted, and it need also RTS rate field to be
set with CTS-to-self rate and RTS duration field to be filled with
CTS-to-self duration.

This patch makes the driver to do this.
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>