samples: led_lp50xx: fix color mapping and channel test

Some boards may have less LED child nodes in the DT than the maximum
number of LEDs supported by the chipset. For these boards, the channel
test must skip the missing LEDs when preparing the color buffer and not
exit in error due to missing info.

Also, some boards may have a color mapping that is different from RGB
and this mapping should be used when providing the buffer to
led_set_color().

Signed-off-by: Mathieu Anquetin <mathieu.anquetin@groupe-cahors.com>