Simple way to downscale from 1080p@60 to 1080i@60
According to the specs on the Canon site your camera can record in 50i/p and 25p 1920x1080. Are you absolutely sure you can only get 60fps on the HDMI? Because that sounds rather weird that you get more FPS on the HDMI than the camera can actually record.

If you set the recording option to 25FPS normally on canon cameras the FPS should also apply to the HDMI output.