hwmon: lps331ap: report data should use 2's complement

hwmon: lps331ap: report data should use 2's complement

BZ: 30050

This patch make sure the report data accurate when pressure or
temperature number is negative -- need do transform from 2's
complement.

Change-Id: Ia907b284e0139e5544863d638e0e7c85e50b8553
Signed-off-by: Chen, Jie <jie.d.chen@intel.com>
Reviewed-on: http://android.intel.com:8080/42021
Reviewed-by: Wang, Zhifeng <zhifeng.wang@intel.com>
Reviewed-by: Du, Alek <alek.du@intel.com>
Tested-by: Wang, Zhifeng <zhifeng.wang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>