net: sonic: return NETDEV_TX_OK if failed to map buffer

NETDEV_TX_BUSY really should only be used by drivers that call
netif_tx_stop_queue() at the wrong moment. If dma_map_single() is
failed to map tx DMA buffer, it might trigger an infinite loop.
This patch use NETDEV_TX_OK instead of NETDEV_TX_BUSY, and change
printk to pr_err_ratelimited.

Fixes: d9fb9f38

 ("*sonic/natsemi/ns83829: Move the National Semi-conductor drivers")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>