[POWERPC] cell: Fix errno for modular spufs_create with invalid neighbour

At present, spu_create with an invalid neighbo(u)r will return -ENOSYS,
not -EBADF, but only when spufs.o is built as a module.

This change adds the appropriate errno, making the behaviour the same
as the built-in case.

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