ieee1394: ohci1394: add PPC_PMAC platform code to driver probe

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=7431


iBook G3 threw a machine check exception and put the display backlight
to full brightness after ohci1394 was unloaded and reloaded.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>