i40evf: ignore bogus messages from FW

Occasionally on shutdown, the FW will hand us a bunch of messages filled
with zeros, which can cause us to spin trying to handle them. Just
ignore these and get on with shutting down.

Change-ID: I347e9648f7153ad5a7b7e0847b87f7aad5f3e0da
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>