net: dsa: b53: Fix ARL register definitions

The ARL {MAC,VID} tuple and the forward entry were off by 0x10 bytes,
which means that when we read/wrote from/to ARL bin index 0, we were
actually accessing the ARLA_RWCTRL register.

Fixes: 1da6df85 ("net: dsa: b53: Implement ARL add/del/dump operations")
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>