i40e: Check PCI_IOV config to avoid compile error

The call to i40e_alloc_vfs needs to be wrapped in CONFIG_PCI_IOV because
the function itself is wrapped in the same conditional compile block.

Change-ID: I663c5f1b85e5cfba0b36da8966f7db1a034f408b
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>