powerpc/eeh: Manage EEH_PE_RECOVERING inside eeh_handle_normal_event()

Currently the EEH_PE_RECOVERING flag for a PE is managed by both the
caller and callee of eeh_handle_normal_event() (among other places not
considered here). This is complicated by the fact that the PE may
or may not have been invalidated by the call.

So move the callee's handling into eeh_handle_normal_event(), which
clarifies it and allows the return type to be changed to void (because
it no longer needs to indicate at the PE has been invalidated).

This should not change behaviour except in eeh_event_handler() where
it was previously possible to cause eeh_pe_state_clear() to be called
on an invalid PE, which is now avoided.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>