Unset CONFIG_ADB_PMU_LED_DISK for ppc.

Why making this change? There is a LED driver that is using this config: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/macintosh/via-pmu-led.c It is exposed as /sys/class/leds/pmu-led:front. If CONFIG_ADB_PMU_LED_DISK is yes then the value of trigger will be "disk-activity" otherwise it will be "none" (the value can be overwritten by writing to /sys/class/leds/pmu-led:front/trigger after boot). I think "none" is a better default because the LED is not used as a disk activity indicator in Mac OS, rather it would pulse slowly when the notebook is in sleep mode. The LED is a bit too bright as a disk activity indicator, someone made a video showing this: https://youtu.be/cbmnuUc-G6c?t=55

For reference, Ubuntu, which used to ship a PowerPC version, is not enabling CONFIG_ADB_PMU_LED_DISK (formerly CONFIG_ADB_PMU_LED_IDE).

Should we change ppc64 as well? I don't know at this point. Note that in the LED driver code it appears that only PowerMac7,2 and PowerMac7,3 are allowed to drive the LED.