leds: trigger: netdev: refactor code setting device name

Move the code into a helper, ready for it to be called at
other times. No intended behaviour change.

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