Fix wrong callback signature

adc.hpp

@@ -156,7 +156,7 @@ class Adc<Cfg, io::P, pin> : public detail::AdcImpl<Cfg> {
 
 template <typename Cfg, InputSource src>
 class Adc<Cfg, InputSource, src> : public detail::AdcImpl<Cfg> {
-	using callback_t = void (*)(uint16_t);
+	using callback_t = void (*)(const uint16_t &);
 
   public:
 	static void init(callback_t callback)