leds: pca955x: Allow zero LEDs to be specified

It's valid to use the PCA955x devices just for GPIOs and not for LEDs.
In this case, as PCA955X_TYPE_GPIO is now equivalent to
PCA955X_TYPE_NONE, remove the test for whether we have any child nodes
specified in the devicetree.

A consequence of this is it's now possible to bind the driver to a
PCA955x device when dynamically instantiated through the I2C subsystem's
`new_device` interface.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>