iwlwifi: mvm: fix TCP CSUM offload with WEP and A000 series

When we enabled TCP checksum offload, we need to tell the
firmware where the IP header starts. If we have an IV, then
we need to adapt that value since the IV is placed before
the SNAP header. This is true only for cases where the
driver adds the IV, not the WEP case in which the IV is
added by the firmware itself.

On A000 devices series, the IV is always added by the
device.

Fix this.

Fixes: 5e6a98dc

 ("iwlwifi: mvm: enable TCP/UDP checksum support for 9000 family")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>