i40evf: set descriptor multiple to 32

Hardware requires descriptors to be allocated in groups of 32.

Change-ID: I752ccc96769d1bd8d3018c004b8aeff464045bf2
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>