staging: comedi: dt282x: fix IRQ assignment for dev->irq.

Here, dev->irq is not assigned with irq. comedi_legacy_detach()
is using dev->irq for release irq and dt282x_attach() is using dev->irq
for initialize comedi_subdevice.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>