tty: n_gsm: fix gsm_mux_net_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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2665,7 +2665,7 @@ static inline void muxnet_put(struct gsm_mux_net *mux_net)
 	kref_put(&mux_net->ref, net_free);
 }
 
-static int gsm_mux_net_start_xmit(struct sk_buff *skb,
+static netdev_tx_t gsm_mux_net_start_xmit(struct sk_buff *skb,
 				      struct net_device *net)
 {
 	struct gsm_mux_net *mux_net = netdev_priv(net);