net: do not name the pointer to struct net_device net

"net" is normally for struct net*, pointer to struct net_device
should be named to either "dev" or "ndev" etc.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>