net: mv643xx_eth: use managed devm_kzalloc

This patch moves shared private data kzalloc to managed devm_kzalloc and
cleans now unneccessary kfree and error handling.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>