iwlwifi: mvm: enable RX offloading with TKIP and WEP

Set the flag that indicates that ICV was stripped on if
this option was enabled in the HW.

Cc: stable@vger.kernel.org # 4.13+
[this is needed for the 9000-series HW to work properly]
Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>