net: stmmac: gmac4+: Not all Unicast addresses may be available

Some setups may not have all Unicast addresses filters available. Check
the number of available filters before trying to setup it.

Fixes: 477286b5

 ("stmmac: add GMAC4 core support")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>