rc_update: further tighten timing requirements for valid data

- any real RC data input will be much faster than 3 Hz, so this is an
easy way to minimize bogus decoded data from propagating

src/modules/rc_update/rc_update.h

@@ -92,6 +92,8 @@ public:
 
 private:
 
+	static constexpr uint64_t VALID_DATA_MIN_INTERVAL_US{1_s / 3}; // assume valid RC input is at least 3 Hz
+
 	void Run() override;
 
 	/**