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