dot11decrypt: Fix WEP decryption
For WPA security association (SA) entries are created on sucessful PTK derivation from 4-way handshake frames. WEP though don't use 4-way handshake frames for key derivation and therefore no SA entry is created. Still WEP decryption implementaton expects to find an SA otherwise the decryption is skipped.
Fix broken WEP decryption by removing the check for an existing SA entry and instead form the SA on first successful decryption.
Add also a test for WEP decryption.
Fixes: v3.3.0rc0-1263-g099d2410 ("dot11decrypt: Avoid allocating SA on packet decryption")