bcm63xx_enet: delete a redundant assignment

In the function bcm_enetsw_probe(), 'ret' will be assigned by
bcm_enet_change_mtu(), so 'ret = 0' make no sense.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>