led: move API functions to LED objects

Actually the LED API functions are handling channel objects. The "led"
parameter passed to the functions is defined as a channel number. This
patch redefines it as a LED number instead.

Note that this patch doesn't change anything for the drivers handling
mono-color (or single channel) LEDs. For the drivers handling multicolor
(or multi-channel) LEDs, such as the lp5562, then the API is broken.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>