staging: comedi: mf6x4: A/D converter uses 2's complement coding

According to the user's manual, the A/D converter uses 2's complement
coding. Use the comedi_offset_munge() helper to convert the data to
the offset binary format used by comedi.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
 drivers/staging/comedi/drivers/mf6x4.c | 5 ++---
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>