sensor: bq274xx: fix few parameter calculations

Fix the calculation for designenergy_mwh, as right now it's using a
float casted straight to an int, which results in the factor rounded
from 3.7 to 3. Also rework both that and taperrate so that they don't
use floating point.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>