[POWERPC] spufs: Update SPU_Status[CISHP] in backing runcntl write

Currently, the kernel may fail to restart a SPE context which
has stopped and been swapped out.

This changes spu_backing_runcntl_write to emulate the real
SPU_Status register exactly.  When the SPU Run Control register
is written with SPU_RunCntl[Run] set to '1', the physical SPU
automatically sets SPU_Status[R] and clears SPU_Status[CISHP].

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>