iavf: allow null RX descriptors

In some circumstances, the hardware can hand us a null receive
descriptor, with no data attached but otherwise valid. Unfortunately,
the driver was ill-equipped to handle such an event, and would stop
processing packets at that point.

To fix this, use the Descriptor Done bit instead of the size to
determine whether or not a descriptor is ready to be processed. Add some
checks to allow for unused buffers.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>