mx35pdk: Fix error handling in board_late_init()

If smc911x_initialize() fails we should return the error immediately.

While at it, also check the error from cpu_eth_init().

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>