gpio-mcp23s08: i2c: auto-select base if no DT match or platform data

The call to gpiochip_add made by this driver is capable of auto-selecting a
base if one is not provided. However, it was not called unless there was
already a DT entry or platform data. This patch calls it even if the base is
not already known so that gpiochip_add can attempt to find a usable base.

Signed-off-by: Daniel M. Weeks <dan@danweeks.net>
Acked-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>