idpf: nullify pointers after they are freed

idpf: nullify pointers after they are freed

rss_data->rss_key needs to be nullified after it is freed.
Checks like "if (!rss_data->rss_key)" in the code could fail
if it is not nullified.

Tested: built and booted the kernel.

Fixes: 83f38f210b85 ("idpf: Fix RSS LUT NULL pointer crash on early ethtool operations")
Signed-off-by: Li Li <boolli@google.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>