[POWERPC] spufs: don't set reserved bits in spu interrupt status

This changes the spu context switch code to not write to reserved bits
of spu interrupt status register.
The architecture book says the reserved fields should be set to zero.

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>