[wlan][sme] Introduce Protection type for client

The protection type will soon add WEP support.
The new enum replaces the previously used Option<Rsna> with
the following mapping:
None => Protection::Open
Some(rsna) => Protection::Rsna(rsna)

Test=Existing tests cover this CL already.

Change-Id: I271013f8df246afc66c56e5baf3471c684a3291d