kselftests: timers: freq-step: Fix build warning

Fixes the following build warning:
freq-step.c: In function ‘main’:
freq-step.c:271:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

By returning the return values from ksft_success/fail.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>