gpio: mockup: fix debugfs handling

The debugfs routines returning pointers can return NULL or error codes
embedded with ERR_PTR(). Check the return values with IS_ERR_OR_NULL().

While we're at it: make the error message more specific so it's not
confused with the one emitted when the top-level gpio-mockup debugfs
directory creation fails.

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