libcamera: media_object: Add functions to entities

Media entities convey information about their main function in the
'function' field of 'struct media_v2_entity'.

Store the main function in the MediaEntity function_ class member and provide
a getter function for that.

While at there update comments, keep the MediaPad description in sync
with the MediaEntity one and remove a stale TODO entry.

Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/media_object.h

@@ -84,6 +84,7 @@ class MediaEntity : public MediaObject
 {
 public:
 	const std::string &name() const { return name_; }
+	unsigned int function() const { return function_; }
 	unsigned int deviceMajor() const { return major_; }
 	unsigned int deviceMinor() const { return minor_; }
 
@@ -103,6 +104,7 @@ private:
 	~MediaEntity();
 
 	std::string name_;
+	unsigned int function_;
 	std::string devnode_;
 	unsigned int major_;
 	unsigned int minor_;