net: sxgbe: fix error handling in init_rx_ring()

There are a couple bugs with the error handling in this function.

1) If we can't allocate "rx_ring->rx_skbuff" then we should call
   dma_free_coherent() but we don't.
2) free_rx_ring() frees "rx_ring->rx_skbuff_dma" and "rx_ring->rx_skbuff"
   so calling it in a loop causes a double free.

Also it was a bit confusing how we sometimes freed things before doing
the goto.  I've cleaned it up so it does error handling in normal kernel
style.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>