Correctly edit air/water temp in dive struct

When changing the values of air/water temp after the first time and save,
it will be reset the intial value.
This is fixed by removing the "select_dc" (which makes the change to the
dive struct instead).

Fixes #457

Signed-off-by: Yousef Hamza <jo.adama.93@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>