NAME
       CloseDevice - terminate access to the audio device

   SYNOPSIS
       CloseDevice(iORequest);
                       A1

   FUNCTION
       The CloseDevice routine notifies the audio device that it will no
       longer be used.  It takes an I/O audio request block (IOAudio) and
       clears the device pointer (io_Device).  If there are any channels
       allocated with the same allocation key (ioa_AllocKey), CloseDevice
       frees (ADCMD_FREE) them. CloseDevice decrements the open count, and if
       it falls to zero and an expunge (Expunge) is pending, the device is
       expunged.

   INPUTS
       iORequest   - pointer to audio request block (struct IOAudio)
               io_Device   - pointer to device node, must be set by (or
                             copied from I/O block set by) open (OpenDevice)
               io_Unit     - bit map of channels to free (ADCMD_FREE) (bits 0
                             thru 3 correspond to channels 0 thru 3)
               ioa_AllocKey- allocation key, used to free channels

   OUTPUTS
       iORequest - pointer to audio request block (struct IOAudio)
               io_Device   - set to -1
               io_Unit     - set to zero