iio: scd4x: check return of scd4x_write_and_fetch

Clang static analysis reports this problem
scd4x.c:474:10: warning: The left operand of '==' is a
  garbage value
  if (val == 0xff) {
      ~~~ ^
val is only set from a successful call to scd4x_write_and_fetch()
So check it's return.

Fixes: 49d22b69 ("drivers: iio: chemical: Add support for Sensirion SCD4x CO2 sensor")
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20220301025223.223223-1-trix@redhat.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/chemical/scd4x.c

@@ -471,12 +471,15 @@ static ssize_t calibration_forced_value_store(struct device *dev,
 	ret = scd4x_write_and_fetch(state, CMD_FRC, arg, &val, sizeof(val));
 	mutex_unlock(&state->lock);
 
+	if (ret)
+		return ret;
+
 	if (val == 0xff) {
 		dev_err(dev, "forced calibration has failed");
 		return -EINVAL;
 	}
 
-	return ret ?: len;
+	return len;
 }
 
 static IIO_DEVICE_ATTR_RW(calibration_auto_enable, 0);