[wlan][common] Remove #![allow(unused)] and fix warnings

This directive includes a lot of undesired things like
allow(unused_imports) and, more improtantly, allow(unused_results).

For temporarily allowing dead code, #[allow(dead_code)] can be used on
per-module basis, although it's likely unnecessary for a library crate.

Change-Id: I74a6de50c83de08ec3af9bad91bff617e585498e