ixgbe: Use pci_vfs_assigned instead of ixgbe_vfs_are_assigned

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

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>