powerpc/eeh: Fix crash in eeh_add_device_early() on Cell

The recent change to the EEH probing causes a crash on Cell because
eeh_ops is NULL.

Check if EEH is enabled and if not bail out.

Fixes: ff57b454

 ("powerpc/eeh: Do probe on pci_dn")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>