aout: don't flush before stop

I always found it weird to always flush explicitly before stopping.

And now, it complicates my gapless work (I could do a workaround by adding an extra variable and prevent flush during the gapless transition, but it feels cleaner with this MR).

If you agree, I'll do a pass on all aout modules to check that stop can be called safely without flush (but the main ones are OK with that).

EDIT: I did a pass on all aout modules. Most aout modules do reset or flush from the stop cb and handle the fact that the flush cb might not be called before the stop cb.

Only 3 modules need a fix (cf. commits).