net: dsa: mv88e6xxx: Use standard helper for broadcast address

Use the conventional declaration style of a MAC address in the
kernel (u8 addr[ETH_ALEN]) for the broadcast address, then set it
using the existing helper.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>