clocksource/drivers/timer-atmel-tcb: Allow selecting first divider

The divider selection algorithm never allowed to get index 0. It was also
continuing to look for dividers, trying to find the slow clock selection.
This is not necessary anymore.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-9-alexandre.belloni@bootlin.com