net: stmmac: dwmac1000: Clear unused address entries

In case we don't use a given address entry we need to clear it because
it could contain previous values that are no longer valid.

Found out while running stmmac selftests.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>