staging: comedi: add a 'readback' member to comedi_subdevice

The analog output hardware in most comedi drivers does not provide a
way to readback to last values written to the channels. In order to
provide an (*insn_read) for the analog output subdevice, the comedi
drivers save the last values for each channel in the private data.

Add a new member, 'readback', to the comedi_subdevice definition to
provide a common way to save these values.

Introduce a comedi core function, comedi_alloc_subdev_readback(), to
allocate the memory needed to save the values. This memory will be
automatically kfree'd when the driver is detached.

Introduce a comedi core function, comedi_readback_insn_read(), that
the comedi drivers can use for the (*insn_read) of a subdevice to
return the saved values for each channel.

This will allow removing the boilerplate in the comedi drivers to
return the saved values. In some drivers it will also allow removing
the private data completely.

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>