staging: comedi: ni_mio_common: fix interrupt handler for dev->write_subdev

There may not be a dev->write_subdev, i.e. an analog output subdevice, that
supports async commands. If it doesn't exist the interrupt/dma will never
be enabled.  Fix ni_E_interrupt() so that the analog output subdevice is
only handled if it exists.

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>