[POWERPC] spufs: Fix return value of spufs_mfc_write

This patch changes spufs_mfc_write() to return
correct size instead of 0.

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>