staging: vt6656: [BUG] iwctl_siwencodeext return if device not open

Don't allow entry to iwctl_siwencodeext if device not open.

This fixes a race condition where wpa supplicant/network manager
enters the function when the device is already closed.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Cc: stable@vger.kernel.org # 3.8+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>