leds: trigger: netdev: expose netdev trigger modes in linux include

Expose netdev trigger modes to make them accessible by LED driver that
will support netdev trigger for hw control.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>