power: pm8921-bms: fix soc repoting for fake battery

There is an issue where the real state of charge is reported
 even when the fake battery parameter is set.

Fix it by checking this parameter just before returning the
 state of charge value.

Change-Id: I3840373a6b52011c165baa79dc8d096a3533b358
Signed-off-by: Patrick Cain <pcain@codeaurora.org> rFu