When the audio input device does not respond to poll(), poll the output device

When the audio input device does not respond to poll(), poll the output device
instead. We don't otherwise poll output devices, as they generally remain in a
partially drained state for most of their operation and thus poll() returns
immediately.

git-svn-id: https://svn.code.sf.net/p/freetel/code@1454 01035d8c-6547-0410-b346-abe4f91aad63