qlcnic: Do not disable SR-IOV when VFs are assigned to VMs

o While disabling SR-IOV when VFs are assigned to VMs causes host crash
  so return -EPERM when user request to disable SR-IOV using pci sysfs in
  case of VFs are assigned to VMs.

Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: Shahed Shaikh <shahed.shaikh@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>