staging: comedi: mf6x4: analog output data does not need extra masking

The comedi core validates that all the data values are <= s->maxdata
before calling the (*insn_write) function. The extra masking is not
needed. Remove 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>