Add support for LED hw control for the netdev trigger.

The trigger on calling set_baseline_state to configure a new mode, will
do various check to verify if hw control can be used for the requested
mode in can_hw_control() function.

It will first check if the LED driver supports hw control for the netdev
trigger, then will use hw_control_is_supported() and finally will call
hw_control_set() to apply the requested mode.

To use such mode, interval MUST be set to the default value and net_dev
MUST be set. If one of these 2 value are not valid, hw control will
never be used and normal software fallback is used.

The default interval value is moved to a define to make sure they are
always synced.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>