[PATCH] PowerMac: force only suspend-to-disk to be valid

For a very long time, echoing 'standby' or 'mem' into /sys/power/state has
killed the machine on powerpc.  This patch fixes that.

This patch adds the .valid callback to pm_ops on PowerMac so that only the
suspend to disk state can be entered.  Note that just returning 0 would
suffice since the upper layers don't pass PM_SUSPEND_DISK down, but we
handle it there regardless just in case that changes.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>