leds: lm36274: use struct led_init_data when registering

By using struct led_init_data when registering we do not need to parse
`label` DT property. Moreover `label` is deprecated and if it is not
present but `color` and `function` are, LED core will compose a name
from these properties instead.

Previously if the `label` DT property was not present, the code composed
name for the LED in the form
  "parent_name::"
For backwards compatibility we therefore set
  init_data->default_label = ":";
so that the LED will not get a different name if `label` property is not
present, nor are `color` and `function`.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>