fix infinite draining of the rate plugin in SND_PCM_NONBLOCK mode

fix infinite draining of the rate plugin in SND_PCM_NONBLOCK mode

The draining function of the rate plugin does not handle properly the
SND_PCM_NONBLOCK case. It can write data to the slave plugin each time the
function is called, but does not update its internal state in order to
reach a stopping condition. Use a last_commit_ptr workaround to reach such
condition.

Signed-off-by: Sylvain BERTRAND <sylvain.bertrand@legeek.net>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>