staging: comedi: rtd520: allow attaching without interrupt support

Interrupts ares only required for the ai subdevice command support.

Allow the driver to attach to the board even if request_irq() fails.
Only hook up the command support if the interrupt is available.

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