i40e: use global pci_vfs_assigned() to replace local i40e_vfs_are_assigned()

There is global funcion pci_vfs_assigned(), so use it instead of composing
local one.

Signed-off-by: Ethan Zhao <ethan.kernel@gmail.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>