This was broken in 2022 with a security fix (61605e0ae5d8f) which
disallowed defaulting to the default TX key if there's no unicast
key. Unfortunately this path was also used by WEP transmit.
To fix it, add a separate check which ensures that WEP is configured
(authtype OPEN, privacy enabled) - then also check if the default TX
key is set and that said key is a WEP key.
Fixes: 61605e0ae5d8f
Locally tested:
- rtwn(4) AP and rtwn(4) STA w/ static WEP keys configured