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

Interrupt support in this driver is optional. Don't fail the attach of the
board if the request_irq() fails.

Only allocate and setup the subdevice for the interrupt 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>