net: stmmac: selftests: Check if filtering is available before running

We need to check if the number of available Hash Filters is enough to
run the test, otherwise we will get false failures.

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>