leds: gpio: Refactor code to use devm_gpiod_get_index_optional()

leds: gpio: Refactor code to use devm_gpiod_get_index_optional()

Instead of checking for the specific error codes, replace
devm_gpiod_get_index() with devm_gpiod_get_index_optional().
In this case we just return all errors to the caller and
simply check for NULL in case if legacy GPIO is being used.
As the result the code is easier to read and maintain.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20231016161005.1471768-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Lee Jones <lee@kernel.org>