Second interrupt is needed only when touchscreen mode is used, so don't
request it unconditionally. This removes the following annoying warning
during boot:

exynos-adc 14d10000.adc: error -ENXIO: IRQ index 1 not found

Fixes: 2bb8ad9b ("iio: exynos-adc: add experimental touchscreen support")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20231009101412.916922-1-m.szyprowski@samsung.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>