[POWERPC] spufs: Don't call spu_run_init from spu_reacquire_runnable

This fixes a major bug which was happening when a SPU thread advances
its execution right after being restored to a SPU.  A potentially
outdated NPC value was being (re)written to the SPU.

So, spu_run_init, in this case, was either not doing anything relevant,
or breaking the execution of the SPU thread.

This fixes a common problem of losing a mailbox write when it was done
to a saved context.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>