gpio: xgene-sb: Tidy up fwnode usage

Since f94277af ("of/platform: Initialise dev->fwnode appropriately"),
of_platform_device_create() already initialises dev->fwnode to that of
the appropriate device_node, so within the driver we shouldn't need to
care whether we probed via DT or ACPI.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>