net: stmmac: selftests: Fix L2 Hash Filter test

With the current MAC addresses hard-coded in the test we can get some
false positives as we use the Hash Filtering method. Let's change the
MAC addresses in the tests to be unique when hashed.

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>