This fixes a regression from 91f42699

This regressions caused the audio converter chain to fail when a chan_mode
conversion was needed but no converter were found.

In the case of AOUT_CHANMODE_DOLBYSTEREO. The dolby converter won't work with a
stereo output. The lowest priority converter still need to handle this
conversion and drop the chan_mode conversion.

Fixes #19646

(cherry picked from commit 31e11f5b

)
Signed-off-by: Thomas Guillem <thomas@gllm.fr>