Executing `asctl set r 96000` no longer results in weird sample rates
being set on the audio devices. SB16 checks for a sample rate between 1
and 44100 Hz, while AC97 implements double-rate support which allows
sample rates between 8kHz and 96kHZ.
This factors out some hardcoded PCMOut registers into a new private
class called AC97Channel, which wraps around a channel's registers and
provides some shared functionality.
No functional changes.
