tools: timers: fix freq average calculation

Delete a duplicate assignment from this function implementation.
The note means ppm is average of the two actual freq samples.
But ppm have a duplicate assignment.

Signed-off-by: Minjie Du <duminjie@vivo.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>