ipa: raspberrypi: Replace void cast with [[maybe_unused]]

The standard way in C++17 to specify that a function or function
argument may be unused it to specify the [[maybe_unused]] attribute.
Replace manual void casts to silence compiler warnings.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/ipa/raspberrypi/controller/rpi/noise.cpp

@@ -27,10 +27,9 @@ char const *Noise::Name() const
 	return NAME;
 }
 
-void Noise::SwitchMode(CameraMode const &camera_mode, Metadata *metadata)
+void Noise::SwitchMode(CameraMode const &camera_mode,
+		       [[maybe_unused]] Metadata *metadata)
 {
-	(void)metadata;
-
 	// For example, we would expect a 2x2 binned mode to have a "noise
 	// factor" of sqrt(2x2) = 2. (can't be less than one, right?)
 	mode_factor_ = std::max(1.0, camera_mode.noise_factor);