staging: vt6656: Fix possible leak in vnt_download_firmware()

When failing to allocate buffer memory, function vnt_download_firmware() goes
through the wrong exit path and fails to release the already requested
firmware. Thus use the correct cleanup. Detected by Coverity CID 1269128.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Reviewed-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>