rtl8180: make CTS-to-self protection work

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>