gpio: grgpio: use a helper variable to store the address of ofdev->dev

Instead of dereferencing the platform device pointer repeatedly, just
store its address in a helper variable.

Link: https://lore.kernel.org/r/20241015131832.44678-3-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>