enic: Fix addr valid check in enic_set_vf_mac

zero mac address is a valid address for VIC dynamic vnic and sriov Vf's.
Fix the check in enic_set_vf_mac appropriately

Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Sujith Sankar <ssujith@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>