net: dsa: qca8k: add QCA8K_ATU_TABLE_SIZE define for fdb access

Add and use QCA8K_ATU_TABLE_SIZE instead of hardcoding the ATU size with
a pure number and using sizeof on the array.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>