power: qpnp-fg: update SRAM data when the charging status changes

Currently, FG SRAM parameters are updated once every 30 seconds.
However, there are cases where the user wants to see the SRAM
parameters getting updated as soon as the charging status
changes e.g. from charging to discharging. Update the SRAM
parameters if the status changes and the last update of SRAM
parameters was done 5 seconds before.

While at it, remove the prev_status defined under learning data
as it is not used at all.

CRs-Fixed: 860061
Change-Id: Id5b97a29cf7d67a317d71726a126060852fba913
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>