ipa: rpi: Make monoSensor() accessor const

The ipa::RPi::IpaBase::monoSensor() function doesn't modify the class.
Make it const.

Fixes: 2031e2f290 ("ipa: rpi: Add accessor function for monoSensor_")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

src/ipa/rpi/common/ipa_base.h

@@ -49,7 +49,7 @@ public:
 	void processStats(const ProcessParams &params) override;
 
 protected:
-	bool monoSensor()
+	bool monoSensor() const
 	{
 		return monoSensor_;
 	}