net: dsa: hellcreek: Use is_zero_ether_addr() instead of memcmp()

Using is_zero_ether_addr() instead of directly use
memcmp() to determine if the ethernet address is all
zeros.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>