mac: re-add stop_ranging functionality

The stop_ranging variable was dropped by commit 891055f7 ("GKI: uwb:
fix unused-variable errors") since it wasn't being used. Now that commit
af7d307f ("mac: add debug traces and mac enhancement") was
introduced by the merge and uses the stop_ranging variable, the code
need to be re-added.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: Icd18bfb6347d8f33a5389cd103ee1f9181bf7097