[POWERPC] EEH: log error only after driver notification.

It turns out many/most versions of firmware enable MMIO when
the slto-error-detail rtas call is made (in violation of the
architecture). Thus, it would be best to call slot-error-detail
only after notifying device drivers of a freeze, as otherwise,
a variety of strange and unexpected things may happen.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>