net: stmmac: xgmac: Not all Unicast addresses may be available

Some setups may not have all Unicast addresses filters available. Let's
check this before trying to setup filters.

Fixes: 0efedbf1

 ("net: stmmac: xgmac: Fix XGMAC selftests")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>