staging: comedi: drivers: digital output subdevices do not need SDF_READABLE

The SDF_READABLE flag is not necessary for digital output subdevices. For
consistency, remove this flag from the comedi drivers that set it.

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>