leds: trigger: netdev: Convert device attr to macro

Convert link tx and rx device attr to a common macro to reduce common
code and in preparation for additional attr.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230419210743.3594-5-ansuelsmth@gmail.com