ET131X uses netdev interfaces so it should depend on NET.

Fixes these build errors:
ERROR: "ethtool_op_get_link" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "eth_validate_addr" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "register_netdev" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_connect" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_find_first" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "mdiobus_register" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "mdiobus_alloc" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "alloc_etherdev_mqs" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "netif_device_detach" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_stop" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "netif_device_attach" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_start" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "free_netdev" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "mdiobus_free" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "mdiobus_unregister" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "unregister_netdev" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "__netif_schedule" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_print_status" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "dev_kfree_skb_any" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "dev_alloc_skb" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "netif_rx" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "eth_type_trans" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "skb_put" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_ethtool_gset" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_ethtool_sset" [drivers/staging/et131x/et131x.ko] undefined!
ERROR: "phy_mii_ioctl" [drivers/staging/et131x/et131x.ko] undefined!
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>