staging: comedi: ni_labpc_common: use subdevice readback for 'caldac'

Use the comedi_subdevice 'readback' member and the core provided (*insn_read)
to handle the readback of the write-only caldac subdevice. Remove the then unused
'caldac' member from the private data.

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>