sensor: kxtj9: Add G-data calculation for 8-bit mode

The driver can not calculate gravity correctly when sensor
working on 8-bit output mode. Add new calculation to handle
this 8-bit output data.

Signed-off-by: Figo Wang <figow@codeaurora.org>
Change-Id: Ia18d7bdf2d8845553062ac9b551ab00711a0a491