net: dsa: simplify the return rtl8366_vlan_prepare()

Simplify the return expression.

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>