gpio: xilinx: Return 0 from xilinx_gpio_set_value

.set_value functions have no specified return value and gpio_uclass is
not working with it too. But this patch is returning 0 to be in sync
with others DM gpio drivers.

Reported-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>