net: mv643xx_eth: Drop the NETDEV_TX_BUSY return path

After adding proper stop/wake thresholds, we can expect a queue to never
be full and drop the NETDEV_TX_BUSY return path. In any case, if the queue
cannot accommodate a TSO packet, the packet would be discarded.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>