iwlwifi: pcie: use iwl_tx_cmd_gen2 and not iwl_tx_cmd

When building AMSDU for gen2, code uses iwl_tx_cmd. The only
updated field is len, which is in the same location, so it
is not a bug. However, it is a bit confusing and error prone,
so change it.
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/intel/iwlwifi/pcie/tx-gen2.c

@@ -238,7 +238,7 @@ static int iwl_pcie_gen2_build_amsdu(struct iwl_trans *trans,
 {
 #ifdef CONFIG_INET
 	struct iwl_trans_pcie *trans_pcie = IWL_TRANS_GET_PCIE_TRANS(trans);
-	struct iwl_tx_cmd *tx_cmd = (void *)dev_cmd->payload;
+	struct iwl_tx_cmd_gen2 *tx_cmd = (void *)dev_cmd->payload;
 	struct ieee80211_hdr *hdr = (void *)skb->data;
 	unsigned int snap_ip_tcp_hdrlen, ip_hdrlen, total_len, hdr_room;
 	unsigned int mss = skb_shinfo(skb)->gso_size;