wl1271: implement cmd_disconnect

This patch implements the CMD_DISCONNECT command, which should be sent to the
firmware when we are disassociated.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>