libcamera: v4l2_subdevice: Breakout mbus code enumeration

Simplify frame size enumeration by breaking out mbus code enumeration in
a helper, making the code easier to read while also preparing for
enhancing the frame size enumeration. There is no functional change.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/include/v4l2_subdevice.h

@@ -57,6 +57,7 @@ protected:
 	std::string logPrefix() const;
 
 private:
+	std::vector<unsigned int> enumPadCodes(unsigned int pad);
 	int enumPadSizes(unsigned int pad, unsigned int code,
 			 std::vector<SizeRange> *size);