staging: comedi: dt3000: don't fail attach if irq is not available

The irq is only needed to support async commands. Don't fail the attach
if it is not available.

Only hook up the command support if the request_irq() was successful.

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>