davinci_emac: Fix use after free in davinci_emac_remove

free_netdev finally calls kfree which makes the contents
of ndev and priv (private data contained in ndev) invalid.

So iounmap should be called before free_netdev.

Cc: David S. Miller <davem@davemloft.net>
Cc: Chaithrika U S <chaithrika@ti.com>
Cc: Sriramakrishnan <srk@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: David S. Miller <davem@davemloft.net>