staging: comedi: dt9812: convert digital out subdevice to (*insn_bits)

Currently the (*insn_write) function for the digital output subdevice
only sets the state for a single channel. It's more efficent to use
the (*insn_bits) function and allow setting the state for all the
channels.

The comedi core can use the (*insn_bits) to emulate the (*insn_write)
if needed.

Also, use the subdevice 'state' variable to hold the current state
of the channels instead of 'do_shadow' in the private data.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>