While upgrading the GTA04 kernel to 3.12-rc4 we came across
an issue with libertas/sdio referencing stale memory on ifconfig up
when trying to load the firmware (for a second time).

I am not at all sure if the patch is how it should be done and the right
location, but it appears to work for us with resetting priv->helper_fw to NULL
before asynchronously loading the firmware again.