staging: comedi: ni_mio_common: don't change CMDF_WRITE flag

There is no need for `ni_ai_cmdtest()` or `ni_ao_cmdtest()` to set the
`CMDF_WRITE` flag to the correct state as it has already been done by
the core comedi module.

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>