gpio: rcar: Cache gpiochip_get_data() return value

Since commit 43c54eca ("gpio: move the subdriver data pointer
into gpio_device") changed gpiochip_get_data() to an out-of-line
function, it is now worthwhile to avoid multiple calls in a row by
caching its return value in a local variable.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>