[POWERPC] EEH: Tolerate high mmio

Some drivers will attempt to perform a lot of mmio even after
an EEH event was detected. This is especially the case for fast cpu's
and PCI-E slots. Be a bit more lenient in allowing this.

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