pcm: plugin status - revert the recent changes

pcm: plugin status - revert the recent changes

It's no reason to sync the avail/delay fields using the mirrored
buffer pointers. The slave information must be valid.

The original report probably tries to fix something for
the specific plugin. Revert all changes.

Fixes: afe6ff3b33e ("pcm: plugin status - fix the return value (regression)")
Fixes: 4f90392f07e ("pcm: fix the snd_pcm_plugin_status() avail and delay fields")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>