staging: comedi: Move pcm do_cmdtest function into a single source file

Many of the comedi source code has functions that were created with
cut and paste, this moves the do_cmdtest function into a single file.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>