i40evf: change version string generation

Generate version strings like the PF driver does. This gives us more
flexibility to add suffixes to the version string at build time.

Change-ID: I0a5ca0783dd8fb849516bfc1e37ea070127847bd
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>