net: stmmac: selftests: Guard VLAN Perfect test against non supported HW

When HW does not support perfect filtering the feature will not be
enabled in the net_device. Add a check for this to prevent failures.

Fixes: 1b2250a0

 ("net: stmmac: selftests: Add tests for VLAN Perfect Filtering")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>