net: dsa: b53: Utilize common helpers for u64/MAC

Utilize the two functions recently introduced: u64_to_ether() and
ether_to_u64() instead of our own versions.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>