leds: trigger: netdev: Rename add namespace to netdev trigger enum modes

Rename NETDEV trigger enum modes to a more symbolic name and add a
namespace to them.

Also add __TRIGGER_NETDEV_MAX to identify the max modes of the netdev
trigger.

This is a cleanup to drop the define and no behaviour change are
intended.

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/20230419210743.3594-4-ansuelsmth@gmail.com