ixgbe: using is_zero_ether_addr() to simplify the code

Using is_zero_ether_addr() to simplify the code.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>