p54: crypto offload fixes

This patch fixes two small flaws:
 - restore the original TKIP IV if we altered it.
 - reserve & initialize ICV with zeros.
   This is actually only necessary for some obsolete p54usb firmwares.
   But we don't know yet, if all devices are compatible with the new revisions.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>