WM8731: Add method to detect codec is present.

Used to determine PortaPack hardware version.

firmware/common/wm8731.cpp

@@ -87,6 +87,10 @@ void WM8731::init() {
 	headphone_mute();
 }
 
+bool WM8731::detected() {
+	return reset();
+}
+
 bool WM8731::reset() {
 	return write(0x0f, 0);
 }

firmware/common/wm8731.hpp

@@ -292,6 +292,8 @@ public:
 
 	bool reset();
 
+	bool detected();
+
 	void set_line_in_volume(const volume_t volume) {
 		const auto normalized = line_in_gain_range.normalize(volume);
 		auto n = normalized.centibel() / 15;