staging: comedi: amplc_dio200_common: remove some tests from amplc_dio200_common_detach()

`amplc_dio200_common_detach()` doesn't do much apart from freeing the
IRQ handler that was requested by `amplc_dio200_common_attach()` if
`dev->irq` is non-zero.  There is no need to check if the pointer to
the constant board data or the pointer to private per-device data
exist, so remove those tests.

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>