net: microchip: sparx5: correctly free skb in xmit

consume_skb on transmitted, kfree_skb on dropped, do not free on
TX_BUSY.

Previously the xmit function could return -EBUSY without freeing, which
supposedly is interpreted as a drop. And was using kfree on successfully
transmitted packets.

sparx5_fdma_xmit and sparx5_inject returns error code, where -EBUSY
indicates TX_BUSY and any other error code indicates dropped.

Fixes: f3cad261

 ("net: sparx5: add hostmode with phylink support")
Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>