mana: Fix TX CQE error handling

For an unknown TX CQE error type (probably from a newer hardware),
still free the mbuf, update the queue tail, etc., otherwise the
accounting will be wrong.

Also, TX errors can be triggered by injecting corrupted packets, so
replace the mana_err to mana_dbg logging.

Reported by: NetApp
MFC after: 1 week
Sponsored by: Microsoft