powerpc/eeh: More relaxed hotplug criterion

Currently, we rely on the existence of struct pci_driver::err_handler
to decide if the corresponding PCI device should be unplugged during
EEH recovery (partially hotplug case). However that check is not
sufficient. Some device drivers implement only some of the EEH error
handlers to collect diag-data. That means the driver still expects a
hotplug to recover from the EEH error.

This makes the hotplug criterion more relaxed: if the device driver
doesn't provide all necessary EEH error handlers, it will experience
hotplug during EEH recovery.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
[mpe: Minor change log rewording]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>