usb: gadget: f_phonet: fix pn_net_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: NLuc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_phonet.c

@@ -221,7 +221,7 @@ static void pn_tx_complete(struct usb_ep *ep, struct usb_request *req)
 	netif_wake_queue(dev);
 }
 
-static int pn_net_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t pn_net_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct phonet_port *port = netdev_priv(dev);
 	struct f_phonet *fp;