i40e: don't free nonexistent rings

Not all VSIs have rings! Check to see if rings were actually allocated before
freeing them.

This prevents a panic when tx_rings[0] is not allocated.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>