qpnp-fg: fix integer overflow

There is a possibility of integer overflow during capacity
learning calculation. Fix this by storing intermediate results
in 64 bit variable.

CRs-Fixed: 2070510
Change-Id: I70b6ef0796be010129f8d8a7380f685bb76daf7d
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>