Support ADT7461 in extended temperature range mode, which will change
the range of readings from 0..127 to -64..191 degC.  Adjust the
register conversion functions accordingly.
Signed-off-by: NNate Case <ncase@xes-inc.com>
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Tested-by: NMartyn Welch <martyn.welch@gefanuc.com>