igb: use eth_zero_addr() to clear mac address

Use eth_zero_addr() to clear mac address instead of memset().

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>