net: stmmac: Fix VLAN filtering when HW does not support it

If we don't have any filters available we can't rely upon the return
code of stmmac_add_hw_vlan_rx_fltr() / stmmac_del_hw_vlan_rx_fltr(). Add
a check for this.

Fixes: ed64639b

 ("net: stmmac: Add support for VLAN Rx filtering")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>