sound/pcm: fix calculation of PCM channel timeout

sound/pcm: fix calculation of PCM channel timeout

this fixes a "pcm0:virtual:0:dsp0.2: play interrupt timeout, channel dead" error
and sound dying after a few seconds of playback.
I encountered on my Thinkpad X40. It was discussed on freebsd-current in 2004[1] but that code is no longer present in FBSD while we still have it.

The patch removes a line fixing a channel timeout to 1 tick instead of the value calculated immediately beforehand, thus restoring the calculation.

[1] http://lists.freebsd.org/pipermail/freebsd-current/2004-September/036805.html