staging: comedi: amplc_dio200_common: prevent extra free_irq()

`dio200_detach()` in "amplc_dio200.c" calls
`amplc_dio200_common_detach()` in "amplc_dio200_common.c", followed by
`comedi_legacy_detach()` in "../drivers.c".  Both of those functions
call `free_irq()` if `dev->irq` is non-zero.  The second call produces a
warning message because the handler has already been freed.  Prevent
that by setting `dev->irq = 0` in `amplc_dio200_common_detach()`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>