powerpc/pci: Fix build with PCI_IOV=y and EEH=n

powerpc/pci: Fix build with PCI_IOV=y and EEH=n

Despite attempting to fix this in commit 83dafb6ea3ad ("powerpc/pci: Fix
SRIOV not building without EEH enabled"), the build is still broken when
PCI_IOV=y and EEH=n (eg. g5_defconfig with PCI_IOV=y):

  arch/powerpc/kernel/pci_dn.c: In function ‘remove_dev_pci_data’:
  arch/powerpc/kernel/pci_dn.c:230:18: error: unused variable ‘edev’

Incorporate Ben's idea of using __maybe_unused to avoid so many #ifdefs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>