i40e: make i40evf_map_rings_to_vectors void

This function cannot fail, so why is it returning a value? And why are
we checking it? Why shouldn't we just make it void? Why is this commit
message made up of only questions?

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>