powerpc/spufs: don't extend time time slice if context is not in spu_run

An spu context shouldn't get an extra tick if the time slice code
couldn't find something else to run. This means contexts that are not
within spu_run (ie, SPU_SCHED_SPU_RUN is cleared) will not receive
extra ticks while we have no other contexts waiting.

Signed-off-by: Luke Browning <lukebrowning@us.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>