media: mgb4: Fix inconsistent input/output alignment in loopback mode

Fixes broken output due to different input/output alignment in loopback
mode when the (last) input device is closed. Instead of on device close,
do the alignment synchronisation when starting the stream (and clear
it when streaming is stopped).

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>