gpiolib-acpi: Remove unnecessary WARN_ON from acpi_gpiochip_free_interrupts

acpi_gpiochip_alloc_event only continues allocating an event and adding
it to the list if gpiochip_request_own_desc does not return an error.
So events with an error desc are never placed on the events list and this
check is really not necessary.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>