ot1200: Fix the error handling in board_eth_init()

We should not return 0 on failure, so return a negative error code
instead.

Also centralize the error path so that is easier to follow.

Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>