gpio-pca953x: fix gpio_base

gpio_base was set to 0 if no system platform data or open firmware
platform data was provided. This led to conflicts, if any other gpiochip
with a gpiobase of 0 was instantiated already. Setting it to -1 will
automatically use the first one available.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>