leds: renesas-tpu: Improve the readability to pick the lowest acceptable rate

I spent a few minutes to understand why the code catching the mismatch case
by checking if k is 0 or not. And the code using "k - 1" as array index is
unusual.

This patch checks acceptable rate from the lowest rate, and then we don't need
to subtract k by 1. This change improves the readability.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>