staging: comedi: pcl816: only init command support if irq is available

The async command support requires an irq. Only initialize the subdevice
command support if the request_irq was successfull.

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>