fix return value of snd_pcm_rewind()/snd_pcm_forward() to return how much actually...

fix return value of snd_pcm_rewind()/snd_pcm_forward() to return how much actually has been rewound, instead of what actually could have rewound

Make snd_pcm_plugin_rewind()/_forward() actually return how much has
been rewound/forwarded instead of how much could have been
rewounded/forwarded. This makes the code actually do what the
documentation of snd_pcm_rewind() suggests.

Signed-off-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>