gpiolib: probe deferral error reporting

Actually report the error code from devm_regulator_get() which may as
well just be a probe deferral.

This is e.g. what one gets upon booting a Colibri T20:

gpiochip_add_data_with_key: GPIOs 0..223 (tegra-gpio) failed to register

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>