power: qpnp-fg: qpnp-qg: fg-alg: Improve code readability of TTF algorithm

For voltage based step charging, used in TTF algorithm, improve the code
readability by changing ttf_mode from TTF_MODE_V_STEP_CHG to
TTF_MODE_VBAT_STEP_CHG.

Change-Id: Ia3b7dd0f2fc1b354c5afaa745c1117719fb9df66
Signed-off-by: Sahil Chandna <chandna@codeaurora.org>