RDMA/hns: Optimize hns_roce_v2_set_mac()

Removes the unnecessary memset opertaion and adjust style of some lines in
hns_roce_v2_set_mac().

Link: https://lore.kernel.org/r/1586938475-37049-3-git-send-email-liweihang@huawei.com


Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>