[POWERPC] cell: catch errors from sysfs_create_group()

We're currently getting a warning from not checking the result of
sysfs_create_group, which is declared as __must_check.

This change introduces appropriate error-handling for
spu_add_sysdev_attr_group()

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