net: stmmac: selftests: Needs to check the number of Multicast regs

When running the MC and UC filter tests we setup a multicast address
that its expected to be blocked. If the number of available multicast
registers is zero, driver will always pass the multicast packets which
will fail the test.

Check if available multicast addresses is enough before running the
tests.

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>