gpio: rcar: Use local variable gpio_chip in gpio_rcar_probe()

The existing variable gpio_chip already points to the gpiochip instance,
hence use it everywhere.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>