alpha: kernel: osf_sys: Set 'kts.tv_nsec' only when 'tv' has effect

The related warning:

    CC      init/do_mounts.o
  arch/alpha/kernel/osf_sys.c: In function 'SyS_osf_settimeofday':
  arch/alpha/kernel/osf_sys.c:1028:14: warning: 'kts.tv_nsec' may be used uninitialized in this function [-Wmaybe-uninitialized]
    kts.tv_nsec *= 1000;
                ^
  arch/alpha/kernel/osf_sys.c:1016:18: note: 'kts' was declared here
    struct timespec kts;
                    ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>