i40e: fix null dereference

If the vsi->tx_rings structure is NULL we don't want to panic.

Change-Id: Ic694f043701738c434e8ebe0caf0673f4410dc10
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>