[POWERPC] spufs: Fix timing dependent false return from spufs_run_spu

Stop bits are only valid when the running bit is not set.  Status bits
carry over from one invocation of spufs_run_spu() to another, so the
RUNNING bit gets added to the previous state of the register which may
have been a remote library call.  In this case, it looks like another
library routine should be invoked, but the spe is actually running.

This fixes a problem with a testcase that exercises the scheduler.

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