staging: comedi: das08: use s->state in das08_do_wbits()

Use the subdevice 'state' variable instead of having to calculate the
current state based on the do_mux_bits in the private data.

Use comedi_dio_update_state() to handle the boilerplate code to update
the subdevice s->state.

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>