staging: comedi: ni_atmio16d: tidy up the irq support in atmio16d_attach()

Tidy up the code that does the request_irq().

Only hookup the commad support if the irq was sucessfully requested.

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>