mac-capture: Detect channels with AudioBufferList

Certain devices such as soundflower 64ch have a lot of unused channels,
so if the device has an unusually large number of channels.  However,
one thing that I did not notice is that it doesn't actually use them
all.  Instead, the audio buffers (in the AudioBuffer struct) have a
mNumberChannels member that I didn't notice.  For things like
soundflower 64ch, it will contain the correct number of channels rather
than the 64 channels it reports on device initialization.