net: stmmac: xgmac: Fix VLAN register handling

Commit 907a0768, forgot that we need to clear old values of
XGMAC_VLAN_TAG register when we switch from VLAN perfect matching to
HASH matching.

Fix it.

Fixes: 907a0768

 ("net: stmmac: xgmac: fix incorrect XGMAC_VLAN_TAG register writting")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>