power: qpnp-fg-gen4: qpnp-fg-gen3: Update TTF variable names

TTF algorithm is getting moved to fg-alg.c so that GEN4 FG driver
can share it with QG driver. In preparation for it, change TTF
variable names so that the namespace won't conflict.

Change-Id: I80d899277692c259182d537533551f9503a7cd3e
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>