treewide: Remove top-level const from return types

Top-level `const` qualifiers are not useful, so avoid them. This is done
either by simply removing the top-level `const`, or making the function
return a reference to const where that is appropriate.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>

src/libcamera/v4l2_subdevice.cpp

@@ -923,7 +923,7 @@ const MediaBusFormatInfo &MediaBusFormatInfo::info(uint32_t code)
  * \brief Assemble and return a string describing the format
  * \return A string describing the V4L2SubdeviceFormat
  */
-const std::string V4L2SubdeviceFormat::toString() const
+std::string V4L2SubdeviceFormat::toString() const
 {
 	std::stringstream ss;
 	ss << *this;