i40evf: change branding string

Add a slash to the branding string to reduce confusion and match up with
our other marketing materials.

Change-ID: I8229e8c3e43083b7a29c859a250f8d2d4dc46b9e
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>