leds: trigger: netdev: reject interval store for hw_control

Reject interval store with hw_control enabled. It's are currently not
supported and MUST be set to the default value with hw control enabled.

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