net: stmmac: xgmac: Detect Hash Table size dinamically

Since commit b8ef7020

 ("net: stmmac: add support for hash table size
128/256 in dwmac4"), we can detect the Hash Table dinamically.

Let's implement this feature in XGMAC cores and fix possible setups that
don't support the maximum size for Hash Table.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>