staging: comedi: addi_apci_3xxx: remove 'b_SingleDiff' private data

The analog reference desired for the acquisition is stored in the
insn->chanspec that is passed to the (*insn_read) function. There is
no need to use a private data variable to hold the desired analog
reference that is set in the (*insn_config).

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>