staging: comedi: pcmuio: tidy up local variables in pcmuio_handle_intr_subdev()

Tidy up the local variables so that the comedi_cmd is accessed as a pointer
instead of getting to it from the comedi_subdevice pointer.

Remove the local variable 'len' and use the cmd->chanlist_len directly.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>