staging: comedi: das800: add a local var for the comedi_cmd pointer

For aesthetics, use a local variable to access the comedi_cmd as a
pointer in das800_ai_do_cmd() instead of getting to it from the
comedi_async pointer.

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>