leds: trigger: netdev: introduce check for possible hw control

Introduce function to check if the requested mode can use hw control in
preparation for hw control support. Currently everything is handled in
software so can_hw_control will always return false.

Add knob with the new value hw_control in trigger_data struct to
set hw control possible. Useful for future implementation to implement
in set_baseline_state() the required function to set the requested mode
using LEDs hw control ops and in other function to reject set if hw
control is currently active.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>