igb: resolve panic on shutdown when SR-IOV is enabled

The setup_rctl call was making a call into the ring structure after it had
been freed.  This was causing a panic on shutdown.  This call wasn't
necessary since it is possible to get the needed index from
adapter->vfs_allocated_count.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>