qmi_wwan: add MTU default to qmap network interface

This patch adds MTU default value to qmap network interface in
order to avoid "RTNETLINK answers: No buffer space available"
error when setting an ipv6 address.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/qmi_wwan.c

@@ -123,6 +123,7 @@ static void qmimux_setup(struct net_device *dev)
 	dev->addr_len        = 0;
 	dev->flags           = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
 	dev->netdev_ops      = &qmimux_netdev_ops;
+	dev->mtu             = 1500;
 	dev->needs_free_netdev = true;
 }