net: ravb: Enforce descriptor type ordering

net: ravb: Enforce descriptor type ordering

Ensure the TX descriptor type fields are published in a safe order so the
DMA engine never begins processing a descriptor chain before all descriptor
fields are fully initialised.

For multi-descriptor transmits the driver writes DT_FEND into the last
descriptor and DT_FSTART into the first. The DMA engine begins processing
when it observes DT_FSTART. Move the dma_wmb() barrier so it executes
immediately after DT_FEND and immediately before writing DT_FSTART
(and before DT_FSINGLE in the single-descriptor case). This guarantees
that all prior CPU writes to the descriptor memory are visible to the
device before DT_FSTART is seen.

This avoids a situation where compiler/CPU reordering could publish
DT_FSTART ahead of DT_FEND or other descriptor fields, allowing the DMA to
start on a partially initialised chain and causing corrupted transmissions
or TX timeouts. Such a failure was observed on RZ/G2L with an RT kernel as
transmit queue timeouts and device resets.

Fixes: 2f45d1902acf ("ravb: minimize TX data copying")
Cc: stable@vger.kernel.org
Co-developed-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://patch.msgid.link/20251017151830.171062-4-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>