gpio: mockup: tweak gpio_mockup_event_write()

Invert the logic of the irq_enabled check and only access the private
data after the input is sanitized.

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>