iio: exynos-adc: request second interupt only when touchscreen mode is used

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>