iio temperature/mlx90632: silence a static checker warning

This shouldn't affect runtime at all, but Smatch complains that we
should check if mlx90632_read_ambient_raw() otherwise we
"ambient_new_raw" can be uninitialized.

    drivers/iio/temperature/mlx90632.c:509 mlx90632_calc_ambient_dsp105()
    error: uninitialized symbol 'ambient_new_raw'.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>