ethernet: sun: Free the coherent when failing in probing

When the driver fails to register net device, it should free the DMA
region first, and then do other cleanup.

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>