[POWERPC] spufs: fail spu_create with invalid flags

At this time, all flags are invalid. Since we are
planning to actually add valid flags in the future,
we better check if any were passed by the user.

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