libertas/sdio: fix releasing memory twice.
We have connected a Wi2Wi W2CBW003 to an OMAP3 using SDIO.
We have seen an issue (not related with this patch) that sometimes power is not
turned off. This did lead to a kernel Oops if an ifconfig up / down / up when the
chip was not powered down. This leads to a second call to lbs_get_firmware_async()
with the same priv data - and that tries to release_firmware(priv->helper_fw);
This appears to be wrong, since it was alredy released in the if_sdio_do_prog_firmware.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>