net_device: Reorder members to fill holes

Some trivial reorders while preserving the RX/TX cache lines
split to fill a couple of holes.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>