staging: comedi: das16m1: remove unnecessary 'dev->irq' test

This function can only be called if the irq was sucessfully requested.
The dev->irq will always be valid.

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>