Allow MTU to be set greater than 1500bytes

[tsifb]

Driver code does not set ndev->max_mtu, so defaults to 1500 bytes.
Wifi will carry packets with larger size, and this is useful when using another protocol to tunnel, while not needing to fragment large IP frames.

The following diff shows a tested & working example, however the max size should be set to a value that is tested to work with the WILC firmware.

index 9c95f2c..c120779 100644
--- a/wilc/wilc_netdev.c
+++ b/wilc/wilc_netdev.c
@@ -1387,6 +1387,7 @@ struct wilc_vif *wilc_netdev_ifc_init(struct wilc *wl, const char *name,
 	ndev->ml_priv = vif;
 	strcpy(ndev->name, name);
 	ndev->netdev_ops = &wilc_netdev_ops;
+	ndev->max_mtu = 1532;
 
 	SET_NETDEV_DEV(ndev, wiphy_dev(wl->wiphy));