MIPS: locking/atomic: Fix atomic{_64,}_sub_if_positive

commit cb95ea79 upstream.

This looks like a typo and that caused atomic64 test failed.

Signed-off-by: Rui Wang <wangrui@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>