[POWERPC] spufs: return an error in spu_create is isolated create isnt supported

This changes the spu_create system call to return an error (-ENODEV) if
and isolated spu context is requested on hardware that doesn't support
isolated mode.

Tested on systemsim with and without isolation support

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>