qlcnic: Fix guest VLAN

o Clear cached vport vlan variable(vp->vlan) in PF on PCI FLR and
  back-channel termination which will allow to configure guest VLAN
  on VF after force off/shut down the guest VM.

Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: Sucheta Chakraborty <sucheta.chakraborty@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>