stmmac: unwind properly in stmmac_dvr_probe()

The original code had a several problems:
*) It had potential null dereferences of "priv" and "res".
*) It released the memory region before it was aquired.
*) It didn't free "ndev" after it was allocated.
*) It didn't call unregister_netdev() after calling stmmac_probe().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>