nitrogen6x: 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: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>