net: lan966x: Add len field to lan966x_tx_dcb_buf

net: lan966x: Add len field to lan966x_tx_dcb_buf

Currently when a frame was transmitted, it is required to unamp the
frame that was transmitted. The length of the frame was taken from the
transmitted skb. In the future we might not have an skb, therefore store
the length skb directly in the lan966x_tx_dcb_buf and use this one to
unamp the frame.
While at this, also arrange the members in lan966x_tx_dcb_buf not to
have any holes.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>