iwlwifi: mvm: don't mess the SNAP header in TSO for non-QoS packets

When we get large sends on non-QoS association, we had a
bug that mangled the SNAP header. Fix that.

Fixes: a6d5e32f ("iwlwifi: mvm: send large SKBs to the transport")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>