[POWERPC] spufs: sched.c cleanups

Various cleanups to sched.c that don't change the global control flow:

 - add kerneldoc comments to various functions
 - add spu_ prefixes to various functions
 - add/remove context from the runqueue in bind/unbind_context as
   it's part of the logical operation
 - add a call to put_active_spu to spu_unbind_contex as it's logically
   part of the unbind operation

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>