leds: trigger: netdev: expose hw_control status via sysfs

Expose hw_control status via sysfs for the netdev trigger to give
userspace better understanding of the current state of the trigger and
the LED.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>