power: qpnp-bms: Improve logic for storing FCC learnt data

BMS module supports 16 (8-bit) battery backed-up registers.
Make sure that while backing-up the new data, always replace
the oldest stored data.

CRs-Fixed: 730047
Change-Id: I5fc406ec24ce242b722926cc1096f7dcaf561656
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>