staging: comedi: quatech_daqp_cs: use comedi_offset_munge() in ao (*insn_write)

Use the comedi_offset_munge() helper to do the offset binary to two's
complement conversion when writing a new analog output value.

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>