i40e: assign correct vector to VF

Correct math error when assigning MSI-X vectors to VFs. The vectors-per-vf
value reported by the hardware already conveniently reports one less than the
actual value.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>