rtl8187: make CTS-to-self protection work

CTS protection was not working properly because the HW needs
RTS flag to be asserted, and it need also 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>