genirq/timings: Optimize the period detection speed

With a minimal period and if there is a period which is a multiple of it
but lesser than the max period then it will be detected before and the
minimal period will be never reached.

 1 2 1 2 1 2 1 2 1 2 1 2
 <-----> <-----> <----->
 <-> <-> <-> <-> <-> <->

In that case, the minimum period is 2 and the maximum period is 5. That
means all repeating pattern of 2 will be detected as repeating pattern of
4, it is pointless to go up to 2 when searching for the period as it will
always fail.

Remove one loop iteration by increasing the minimal period to 3.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: andriy.shevchenko@linux.intel.com
Link: https://lkml.kernel.org/r/20190527205521.12091-4-daniel.lezcano@linaro.org