staging: comedi: das16: remove unnecessary test in das16_interrupt()

The timer is only started by the ai subdevice in das16_cmd_exec()
and this function can only be called if the device is attached.

Remove the unnecessary sanity check.

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>