leds: pca955x: Let the core process the fwnode

Much of the fwnode processing in the PCA955x driver is now in the
LEDs core driver, so pass the fwnode in the init data when
registering the LED device. In order to preserve the existing naming
scheme, check for an empty name and set it to the LED number.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>