powerpc/powernv/npu: Fix oops in pnv_try_setup_npu_table_group()

powerpc/powernv/npu: Fix oops in pnv_try_setup_npu_table_group()

With a recent change around IOMMU group, a system with an opencapi
adapter is no longer booting and we get a kernel oops:

  BUG: Kernel NULL pointer dereference at 0x00000028
  Faulting instruction address: 0xc0000000000aa38c
  ...
  NIP pnv_try_setup_npu_table_group+0x1c/0x1a0
  LR  pnv_pci_ioda_fixup+0x1f8/0x660
  Call Trace:
    pnv_try_setup_npu_table_group+0x60/0x
    pnv_pci_ioda_fixup+0x20c/0x660
    pcibios_resource_survey+0x2c8/0x31c
    pcibios_init+0xb0/0xe4
    do_one_initcall+0x64/0x264
    kernel_init_freeable+0x36c/0x468
    kernel_init+0x2c/0x148
    ret_from_kernel_thread+0x5c/0x68

An opencapi device is using a device PE, so the current code breaks
because pe->pbus is not defined.

More generally, there's no need to define an IOMMU group for opencapi,
as the device sends real addresses directly (admittedly, the
virtualization story is yet to be written). So let's fix it by
skipping the IOMMU group setup for opencapi PHBs.

Fixes: 0bd971676e68 ("powerpc/powernv/npu: Add compound IOMMU groups")
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>