button: adc: fix treshold typo

Fix the treshold typo in code by threshold.

Fixes: c0165c85 ("button: add a simple Analog to Digital Converter device based button driver")
Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>