PCI: Don't look for ACPI hotplug parameters if ACPI is disabled

PCI: Don't look for ACPI hotplug parameters if ACPI is disabled

Booting a v3.18 or newer Xen domU kernel with PCI devices passed through
results in an oops (this is a 32-bit 3.13.11 dom0 with a 64-bit 4.4.0
hypervisor and 32-bit domU):

  BUG: unable to handle kernel paging request at 0030303e
  IP: [<c06ed0e6>] acpi_ns_validate_handle+0x12/0x1a
  Call Trace:
   [<c06eda4d>] ? acpi_evaluate_object+0x31/0x1fc
   [<c06b78e1>] ? pci_get_hp_params+0x111/0x4e0
   [<c0407bc7>] ? xen_force_evtchn_callback+0x17/0x30
   [<c04085fb>] ? xen_restore_fl_direct_reloc+0x4/0x4
   [<c0699d34>] ? pci_device_add+0x24/0x450

Don't look for ACPI configuration information if ACPI has been disabled.

I don't think this is the best fix, because we can boot plain Linux (no
Xen) with "acpi=off", and we don't need this check in pci_get_hp_params().
There should be a better fix that would make Xen domU work the same way.
The domU kernel has ACPI support but it has no AML.  There should be a way
to initialize the ACPI data structures so things fail gracefully rather
than oopsing.  This is an interim fix to address the regression.

Fixes: 6cd33649fa83 ("PCI: Add pci_configure_device() during enumeration")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=96301
Reported-by: Michael D Labriola <mlabriol@gdeb.com>
Tested-by: Michael D Labriola <mlabriol@gdeb.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.18+