Implement hw_control ops to drive Switch LEDs based on hardware events.

Netdev trigger is the declared supported trigger for hw control
operation and supports the following mode:
- tx
- rx

When hw_control_set is called, LEDs are set to follow the requested
mode.
Each LEDs will blink at 4Hz by default.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>