sys/sunaudio/: Export functions that are needed in other parts of the code, makes...

sys/sunaudio/: Export functions that are needed in other parts of the code, makes the mixer actually work; adjust mag...

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* sys/sunaudio/gstsunaudiomixerctrl.c:
* sys/sunaudio/gstsunaudiosink.c: (gst_sunaudiosink_init):
Export functions that are needed in other parts of the code,
makes the mixer actually work; adjust magic minimum buffer-time
value from 3ms to 5ms to work around stuttering during mp3
playback (#327765).