gpio: mockup: don't return magic numbers from probe()

When the requested number of GPIO lines is 0, return -EINVAL, not
-1 which is -EPERM.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>