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: NMitch Williams <mitch.a.williams@intel.com>
Signed-off-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: NSibai Li <sibai.li@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>