net: stmmac: dwmac4: mac address array boudary violation issue

The mac address array size is GMAC_MAX_PERFECT_ADDRESSES,
so the 'reg' should be less than it, or will affect other registers.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>