leds: Allow to call led_classdev_unregister() unconditionally

If in the certain driver the LED is optional, and it's a majority of them,
the call of led_classdev_unregister() still requires some additional
checks.

The usual pattern on unregistering is to check for NULL, but we also check
for IS_ERR() in case device_create_with_groups() fails.

The change will reduce a burden in a lot of drivers to repeatedly check
for above conditions.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>