iio: health: max30102: Prepare for copying varying number of measurements

Current code assumes always 2 measurements (6 bytes) have to be copied,
prepare for more flexibility

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>