iio: mpl3115: Use scan_type.shift and realbit in mpl3115_read_raw

When processing raw data using channel scan_type.shift as source of
trust to shift data appropriately.
When processing the temperature channel, use a 16bit big endian variable
as buffer to increase conversion readability.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20211104082413.3681212-14-gwendal@chromium.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>