qtnfmac: fix qtnf_netdev_hard_start_xmit()'s return type

The method ndo_start_xmit() is defined as returning an 'netdev_tx_t',
which is a typedef for an enum type, but the implementation in this
driver returns an 'int'.

Fix this by returning 'netdev_tx_t' in this driver too.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Reviewed-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/quantenna/qtnfmac/core.c

@@ -76,7 +76,7 @@ static int qtnf_netdev_close(struct net_device *ndev)
 
 /* Netdev handler for data transmission.
  */
-static int
+static netdev_tx_t
 qtnf_netdev_hard_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 {
 	struct qtnf_vif *vif;