net: fec_mxc_init(): do not ignore return status of fec_open()

net: fec_mxc_init(): do not ignore return status of fec_open()

The fec_mxc_init() function currently always returns 0. This does not
allow the callers to detect when for instance the PHY initialization
failed due to the port being unconnected. Fix that by returning the
status of fec_open().

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>