staging: comedi: das800: tidy up das800_do_insn_bits()

Use a couple local variables, mask and bits, to clarify this function.

Its only necessary to update the outputs if the mask indicates that
the bits are changing. Modify this function accordingly. Also, use
the subdevice 'state' variable to hold the actual output channel
state instead of needing to get it from the private data and shift
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>