leds: pca9532: Assign gpio base dynamically

When using devicetree, gpio_base holds its initial zero value which can
lead to a rejection if another gpio controller already occupies this
base. To prevent that collision let the gpio base be assigned dynamically.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Pavel Machek <pavel@ucw.cz>