net: stmmac: selftests: Prevent false positives in filter tests

In L2 tests that filter packets by destination MAC address we need to
prevent false positives that can occur if we add an address that
collides with the existing ones.

To fix this, lets manually check if the new address to be added is
already present in the NIC and use a different one if so. For Hash
filtering this also envolves converting the address to the hash.

Fixes: 091810db ("net: stmmac: Introduce selftests support")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>