[homematic] Fix UI enumeration of HM-MOD-EM-8 channels (#10907)

[homematic] Fix UI enumeration of HM-MOD-EM-8 channels (#10907)

The set of available HM-MOD-EM-8 channels varies depending on the
function a given channel is configured to use, which is why for those
devices we can't determine a static ThingType, but instead must populate
the thing channels on initialization. The existing code already handled
that case, but missed registering channel types for the dynamically
generated channels, which is why those channels were not shown in main
UI.

Signed-off-by: Danny Baumann <dannybaumann@web.de>