gpio: syscon: do not report bogus error

gpio: syscon: do not report bogus error

Do not issue "can't read the data register offset!" when gpio,syscon-dev
is not set albeit unneeded. gpio-syscon is used with rk3328 chip, but
this iomem region is documented in
Documentation/devicetree/bindings/gpio/rockchip,rk3328-grf-gpio.yaml and
does not require gpio,syscon-dev setting.

Signed-off-by: Etienne Buira <etienne.buira@free.fr>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/Zktwd4Y8zu6XSGaE@Z926fQmE5jqhFMgp6
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>