ethernet: hisilicon: hns: hns_dsaf_mac: Use generic eth_broadcast_addr

Rather than use an on-stack array to copy a broadcast address, use
the generic eth_broadcast_addr function to save a trivial amount of
object code.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>