gpio: mockup: be quiet unless something goes wrong

When inserting and removing the module repeatedly (e.g. when running
the libgpiod test-suite) the kernel log gets clobbered with messages
reporting successful creation of dummy gpiochips.

Remove this message and only emit logs when something bad happens.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>