iio: health: max30102: Move mode setting to buffer_postenable

Move the programming of the mode setting from init() to
buffer_postenable()

Split out a separate function to
only update the power/shutdown bit

This changes permits to more easily implement different
modes of measurements in further patches

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>