iio: health: max30102: Add power enable parameter to get_temp function

Chip must not be in shutdown for reading temperature, so briefly leave
shutdown if buffer is not already running

Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>