edmac_channels for 600D

Created originally on Bitbucket by escho

Was already merged in Bitbucket before import, marked as merged by the import user

There are not so many "free" edmac-channels, which can be used for 600D. Most of them result in corrupted frames (plain white).

With write channel 0x21 I didn´t notice any more corrupted frames.

Testet in normal and crop mode, max possible resolutions and resolutions for continous recording. Using autofocus during recording results in a null pointer error, but no corrupted frames. Silent pics work with no problems.