igb: Use pci_vfs_assigned instead of igb_vfs_are_assigned

This change makes it so that the igb driver uses the generic helper
pci_vfs_assigned instead of the igb specific function igb_vfs_are_assigned.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>