wl1271: added missing command header in wl1271_cmd_disconnect

The command header was missing in the wl1271_cmd_disconnect structure.  It
was working fine by sheer luck, because the parameters are not critical and
because our wl1271_cmd_send() function was overwriting the rx_config_options
with the actual header.

This patch adds the header to the command structure.

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>