[POWERPC] spufs: fix support for read/write on cntl

This fixes a memory leak introduced by "spufs: add support
for read/write oncntl", which was missing a call to simple_attr_close.

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>