CAN: Vector: Preserve the FD flag on short frames

The VectorCAN receive path uses the FD event format for both classic
and FD frames. QCanBusFrame sets the FD flag automatically when the
payload exceeds 8 bytes or BRS is set. Short FD frames without BRS
were therefore exposed as classic frames. Set the flag from the
driver's EDL bit.

[ChangeLog][CAN][VectorCAN] Received short CAN FD frames without
bitrate switching now retain their CAN FD flag.

Pick-to: 6.12 6.8
Assisted-by: OpenAI Codex
Change-Id: I6708a566fdb9914a528797bd9fa1f225584fb845
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>