prevent the sound thread from consuming all the available CPU

prevent the sound thread from consuming all the available CPU
doing busy-wait on the output audio device.
As it is set now, it tries to push a frame every 10ms,
which is still too frequent but avoids deep restructuring
of the code (which i should do, though).

Note, this is only for ring tones, regular audio coming
from the network is still delivered as soon as it is
available.

Eventually this could well end up in the 1.4 branch, but
since i am probably the only user of chan_oss there isn't
much urgency to do that.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@47822 65c4cc65-6c06-0410-ace0-fbb531ad65f3