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>

drivers/sensor/bq274xx/bq274xx.c

@@ -139,8 +139,8 @@ static int bq274xx_gauge_configure(const struct device *dev)
  terminatevolt_msb, terminatevolt_lsb, taperrate_msb, taperrate_lsb;
  uint8_t block[BQ27XXX_DM_SZ];
 
- designenergy_mwh = (uint16_t)3.7 * config->design_capacity;
- taperrate = (uint16_t)config->design_capacity / (0.1 * config->taper_current);
+ designenergy_mwh = (uint32_t)config->design_capacity * 3700 / 1000;
+ taperrate = config->design_capacity * 10 / config->taper_current;
 
  /* Unseal the battery control register */
  ret = bq274xx_ctrl_reg_write(dev, BQ274XX_UNSEAL_KEY);