Return current measurement result in lx.

def _get_result(self) -> float:
        """Return current measurement result in lx."""
        try:
            data = self._bus.read_word_data(self._i2c_add, self._mode)
            self._ok = True
        except OSError as exc:
            self.log_error("Bad reading in bus: %s", exc)
            self._ok = False
            return -1

        count = data >> 8 | (data & 0xff) << 8
        mode2coeff = 2 if self._high_res else 1
        ratio = 1 / (1.2 * (self._mtreg / 69.0) * mode2coeff)
        return ratio * count