leds: pca963x: 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
  "pca963x:%d:%.2x:%u"
For backwards compatibility we therefore set init_data->default_label
to this value 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>
Cc: Peter Meerwald <p.meerwald@bct-electronic.com>
Cc: Ricardo Ribalda <ribalda@kernel.org>
Cc: Zahari Petkov <zahari@balena.io>
Signed-off-by: Pavel Machek <pavel@ucw.cz>