staging: comedi: das800: allow attaching without interrupt support

Interrupts are only requires for the analog input async command support.

Allow the driver to attach to the board if the request_irq() fails.

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>