powerpc/cell: Add missing error code in spufs_mkgang()

We should return -ENOMEM if alloc_spu_gang() fails.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>