power: fg-alg: Use iterm_delta only during CC mode

Currently, iterm_delta is used in both CC/CV charging mode. To
reduce the error in calculations, use iterm_delta only during
CC mode and use the actual iterm alone in CV mode.

Change-Id: Ic9883e64a984881d82ab780c7501924292c36f0d
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>