staging: comedi: das800: interrupts are required for async command support

Interrupt support is optional with this driver. Only hook up the async
command support for the analog input subdevice if the user requested an
interrupt when attaching to the board. This also allows removing the
extra test in das800_ai_do_cmd().

For aesthetic reasons, add some whitespace to the subdevice init.

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>