iwlwifi: mvm: Support TKIP on gen2 data path

Make the adjustments for gen2 TX and RX of TKIP packets.  Strip MIC on
RX.  Don't add IV space and keep the MIC space zeroed on TX.

Devices that support gen2 data path support TKIP only in station mode.
In all other modes, fall back to SW encryption. Do this early in the
set_key() callback so that the key flags would not be incorrectly set.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>