Staging: comedi: pcl816: update convert_src mask for AI cmdtest

The COMEDI_CMDTEST ioctl needs to clear unsupported bits in the
struct comedi_cmd's convert_src and other *_src members.  This
needs fixing in the pcl816 driver's AI cmdtest.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>